Skip default parameters that are null, to avoid NPE when building Environment

[MartinCon]

When a Jenkins job has a parameter of a type that doesn't have a default value, the GHPRB plugin will throw a NullPointerException.

The declaration of hudson.model.ParameterDefinition.getDefaultParameterValue() explicitly allows null as a possible value by having the @CheckForNull annotation.

Such a parameter will result in null being added to the the list returned by GhprbTrigger.getDefaultParameters(), which is then stored in the GhprbParametersAction, which in turn will trigger an NPE when the environment is build.

Here the full stacktrace:

java.lang.NullPointerException
    at org.jenkinsci.plugins.ghprb.GhprbParametersAction$GhprbAdditionalParameterEnvironmentContributor.buildEnvironmentFor(GhprbParametersAction.java:64)
    at hudson.model.Run.getEnvironment(Run.java:2361)
    at org.jenkinsci.plugins.workflow.job.WorkflowRun.getEnvironment(WorkflowRun.java:512)
    at com.continental.jenkinsci.plugin.scm.DevGateSCM.checkout(DevGateSCM.java:511)
    at org.jenkinsci.plugins.workflow.steps.scm.SCMStep.checkout(SCMStep.java:113)
    at org.jenkinsci.plugins.workflow.steps.scm.SCMStep$StepExecutionImpl.run(SCMStep.java:85)
    at org.jenkinsci.plugins.workflow.steps.scm.SCMStep$StepExecutionImpl.run(SCMStep.java:75)
    at org.jenkinsci.plugins.workflow.steps.AbstractSynchronousNonBlockingStepExecution$1$1.call(AbstractSynchronousNonBlockingStepExecution.java:47)
    at hudson.security.ACL.impersonate(ACL.java:290)
    at org.jenkinsci.plugins.workflow.steps.AbstractSynchronousNonBlockingStepExecution$1.run(AbstractSynchronousNonBlockingStepExecution.java:44)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)