Do not look further if nodefilter.dispatch is not set

Solves job import issue described in https://github.com/rundeck/rundeck/issues/1009
If job description does not have 'distpatch' section or dispatch is null, import will fail with exception: 
Caused by: java.lang.NullPointerException: Cannot get property 'threadcount' on null object
	at rundeck.ScheduledExecution.fromMap(ScheduledExecution.groovy:280)
	at rundeck.codecs.JobsYAMLCodec$_createJobs_closure1.doCall(JobsYAMLCodec.groovy:48)

With this change job will be imported successfully with nodefilter.dispatch parameters using default values.