NullPointerException after calling triggerJobAndWaitUntilFinished

When I call `triggerJobAndWaitUntilFinished()` I get an NPE:

```
java.lang.NullPointerException: null
at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:878) ~[guava-28.1-jre.jar:na]
at com.google.common.net.PercentEscaper.escape(PercentEscaper.java:145) ~[guava-28.1-jre.jar:na]
at com.offbytwo.jenkins.model.Job$MapEntryToQueryStringPair.apply(Job.java:185) ~[jenkins-client-0.3.8.jar:0.3.8]
at com.offbytwo.jenkins.model.Job$MapEntryToQueryStringPair.apply(Job.java:181) ~[jenkins-client-0.3.8.jar:0.3.8]
at com.google.common.collect.Iterators$6.transform(Iterators.java:783) ~[guava-28.1-jre.jar:na]
at com.google.common.collect.TransformedIterator.next(TransformedIterator.java:47) ~[guava-28.1-jre.jar:na]
at org.apache.commons.lang.StringUtils.join(StringUtils.java:2831) ~[commons-lang-2.3.jar:2.3]
at org.apache.commons.lang.StringUtils.join(StringUtils.java:2878) ~[commons-lang-2.3.jar:2.3]
at com.offbytwo.jenkins.model.Job.build(Job.java:150) ~[jenkins-client-0.3.8.jar:0.3.8]
at com.offbytwo.jenkins.model.Job.build(Job.java:125) ~[jenkins-client-0.3.8.jar:0.3.8]
at com.offbytwo.jenkins.JenkinsTriggerHelper.triggerJobAndWaitUntilFinished(JenkinsTriggerHelper.java:88) ~[jenkins-client-0.3.8.jar:0.3.8]
...
```
The stacktrace was truncated because the other lines only show my code.

In my code I basically do this:

Map params = new HashMap<>();
params.put("identifier", command.getIdentifier());
...
BuildWithDetails details = null;
try {
details = th.triggerJobAndWaitUntilFinished(jp.getJob(), params, true);
} catch (HttpResponseException e) {
log.warn("You were denied by Jenkins. Please check user and password used", e);
} catch (IOException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (NullPointerException e) {
log.warn("It looks like calling job {} failed. Value of details: {}", jp.getJob(), details, e);
}

I already forced the use of a current version of Guava because by default v17 was chosen. After checking the `params` I found out that in some situations one of the parameters turns `null`. And this causes the library to fail.

Is it possible to have the library return a descriptive error message in such situations ?

But even after I fixed this and made sure only to call the method when no null values are inside the map, the call fails with another NPE:

```
java.lang.NullPointerException: null
at com.offbytwo.jenkins.JenkinsServer.getBuild(JenkinsServer.java:824) ~[jenkins-client-0.3.8.jar:0.3.8]
at com.offbytwo.jenkins.JenkinsTriggerHelper.triggerJobAndWaitUntilFinished(JenkinsTriggerHelper.java:178) ~[jenkins-client-0.3.8.jar:0.3.8]
at com.offbytwo.jenkins.JenkinsTriggerHelper.triggerJobAndWaitUntilFinished(JenkinsTriggerHelper.java:90) ~[jenkins-client-0.3.8.jar:0.3.8]
...
```

I tried to debug this but all I could find out is that here the `queueItem` becomes `null`:


When checking the Jenkins job I can see that it was created (it failed with exit code 6 due to some other reasons, but it was created and ran)