NPE unless branchFilterType is set - Job DSL issue

## Issue

### Context
- **Gitlab plugin version**: 1.5.10
- **Gitlab version**: 1.10.3
- **Jenkins version**: 2.121.1
- **Job type**: Pipeline

### Logs & Traces

```
WebHook called with url: /project//
Error while serving http:///project//
java.lang.NullPointerException
at com.dabsquared.gitlabjenkins.trigger.filter.BranchFilterFactory.newBranchFilter(BranchFilterFactory.java:15)
at com.dabsquared.gitlabjenkins.GitLabPushTrigger.initializeBranchFilter(GitLabPushTrigger.java:492)
at com.dabsquared.gitlabjenkins.GitLabPushTrigger.onPost(GitLabPushTrigger.java:434)
at com.dabsquared.gitlabjenkins.webhook.build.PushBuildAction$1.performOnPost(PushBuildAction.java:86)
at com.dabsquared.gitlabjenkins.webhook.build.BuildWebHookAction$TriggerNotifier.run(BuildWebHookAction.java:54)
at hudson.security.ACL.impersonate(ACL.java:273)
at com.dabsquared.gitlabjenkins.webhook.build.PushBuildAction.execute(PushBuildAction.java:83)
at com.dabsquared.gitlabjenkins.webhook.build.BuildWebHookAction.execute(BuildWebHookAction.java:31)
at com.dabsquared.gitlabjenkins.webhook.GitLabWebHook.getDynamic(GitLabWebHook.java:44)
at java.lang.invoke.MethodHandle.invokeWithArguments(MethodHandle.java:627)
at org.kohsuke.stapler.Function$MethodFunction.invoke(Function.java:343)
Caused: java.lang.reflect.InvocationTargetException
at org.kohsuke.stapler.Function$MethodFunction.invoke(Function.java:347)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:184)
at org.kohsuke.stapler.MetaClass$10.dispatch(MetaClass.java:372)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:715)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:845)
at org.kohsuke.stapler.MetaClass$10.dispatch(MetaClass.java:374)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:715)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:845)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:649)
at org.kohsuke.stapler.Stapler.service(Stapler.java:238)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:860)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1650)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:154)
at org.jenkinsci.plugins.ssegateway.Endpoint$SSEListenChannelFilter.doFilter(Endpoint.java:243)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151)
at io.jenkins.blueocean.ResourceCacheControl.doFilter(ResourceCacheControl.java:134)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151)
at io.jenkins.blueocean.auth.jwt.impl.JwtAuthenticationFilter.doFilter(JwtAuthenticationFilter.java:61)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151)
at hudson.plugins.greenballs.GreenBallFilter.doFilter(GreenBallFilter.java:59)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:157)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at com.dabsquared.gitlabjenkins.webhook.GitLabWebHook$GitlabWebHookCrumbExclusion.process(GitLabWebHook.java:53)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:73)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
at hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:117)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:135)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.BasicHeaderProcessor.doFilter(BasicHeaderProcessor.java:93)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:67)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:90)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:171)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:49)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:82)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at org.kohsuke.stapler.DiagnosticThreadNameFilter.doFilter(DiagnosticThreadNameFilter.java:30)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:533)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:524)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:190)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1595)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:188)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1253)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:168)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:473)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1564)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:166)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1155)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
at org.eclipse.jetty.server.Server.handle(Server.java:530)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:347)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:256)
at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:279)
at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:102)
at org.eclipse.jetty.io.ChannelEndPoint$2.run(ChannelEndPoint.java:124)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:247)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.produce(EatWhatYouKill.java:140)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:131)
at org.eclipse.jetty.util.thread.ReservedThreadExecutor$ReservedThread.run(ReservedThreadExecutor.java:382)
at winstone.BoundedExecutorService$1.run(BoundedExecutorService.java:77)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
```

### Problem description

_Describe your problem in a meaningful way_:

We are running a setup where Jenkins jobs are set up to be triggered by webhooks on a push to every branch in a repo. We recently began trying to configure the job triggers using the Jenkins DSL plugin to create a job where the triggers are configured in an SCM provided Jenkinsfile instead of configuring the triggers manually. When doing so, we ran into an issue where we would run the pipeline manually once to allow the job to pull in the trigger information.

Example trigger config:
```
gitlab(triggerOnPush: true, triggerOnMergeRequest: false, triggerOnNoteRequest: true, noteRegex: "jenkins rebuild")
```

The triggers would then show up as being configured in the web UI. When we pushed to the Github repo, the webhook would show up as having been received by Jenkins in the logs but the job itself had not triggered. After cranking up the debug logging as explained in the README, we stumbled upon the above NPE. Since the NPE appeared to be caused by a null BranchFilterConfig, I changed the triggers config to
```
gitlab(triggerOnPush: true, triggerOnMergeRequest: false, triggerOnNoteRequest: true, noteRegex: "jenkins rebuild", branchFilterType: 'All')
```
and the Jenkins jobs were successfully triggered after running the job manually to pick up the new trigger config. I tried tracking down where the config type was not being set correctly but I'm not much good with Java. :\

The Gitlab webhook is configured as follows:
```
{
"id": "", # snipped
"url": "http:///project//", # snipped
"push_events": true,
"push_events_branch_filter": "",
"issues_events": false,
"confidential_issues_events": false,
"merge_requests_events": false,
"tag_push_events": false,
"note_events": false,
"job_events": false,
"pipeline_events": false,
"wiki_events": false,
"enable_ssl_verification": true,
"token": ""
}
```

Let me know if you need any more info.