Generate service with -l jaxrs-spec fails with NPE

Description

I downloaded one of the latest 3.0 snapshot builds and tried to generate with -l jaxrs-spec, but I got an NPE

Exception in thread "Thread-1" java.lang.NullPointerException
at io.swagger.codegen.languages.AbstractJavaJAXRSServerCodegen.preprocessOpenAPI(AbstractJavaJAXRSServerCodegen.java:107)
at io.swagger.codegen.languages.JavaJAXRSSpecServerCodegen.preprocessOpenAPI(JavaJAXRSSpecServerCodegen.java:161)
at io.swagger.codegen.DefaultGenerator.configureGeneratorProperties(DefaultGenerator.java:199)
at io.swagger.codegen.DefaultGenerator.generate(DefaultGenerator.java:715)
at io.swagger.codegen.cmd.Generate.run(Generate.java:312)
at java.lang.Thread.run(Thread.java:745)

Swagger-codegen version

swagger-codegen-cli-3.0.0-20180111.055017-19.jar

Swagger declaration file content or url

Use any v3 json or yaml sample

Command line used for generation

java -jar swagger-codegen-cli-3.0.0-20180111.055017-19.jar generate -i petstore3.json -l jaxrs-spec

Steps to reproduce

Related issues/PRs

Suggest a fix/enhancement

In AbstractJavaJAXRSServerCodegen, should consider that an operation contained within the operations array can be null.
When creating the operations array with ModelUtils.createOperationArray(pathItem); and passing in only one pathitem, then it will be expected that some of the operations will be null.

Start in AbstractJavaJAXRSServerCodegen.preprocessOpenAPI at the stack-trace line, then inspect JavaJAXRSSpecServerCodegen and ModelUtils.createOperationArray(pathItem). Reproduce with the supplied generate command and petstore3.json or another OpenAPI v3 sample; done means -l jaxrs-spec completes without the reported NullPointerException.