jjb: Fix crash when launching new jobs from the the system trigger

jjb: Fix crash when launching new jobs from the the system trigger

ChoiceParameterDefinition does not have a method getDefaultValue().
However, getDefaultParameterValue() will return null if there is no
default, so the check can be altered.

C.f. https://javadoc.jenkins-ci.org/hudson/model/ParameterDefinition.html
C.f. https://javadoc.jenkins-ci.org/hudson/model/ChoiceParameterDefinition.html

Original crash:

```
ERROR: Build step failed with exception
groovy.lang.MissingMethodException: No signature of method:
hudson.model.ChoiceParameterDefinition.getDefaultValue() is applicable
for argument types: () values: []
...
	at Script1$_run_closure8.doCall(Script1.groovy:276)
...
	at Script1$_run_closure14.doCall(Script1.groovy:434)
...
```

Change-Id: I8847806b2abb556ff2b89aa772277b1318c507e1