jjb: Fix crash when trying to launch a job that is not defined

jjb: Fix crash when trying to launch a job that is not defined

Original error:

```
Launching job: baremetal_tests_kmaster_lmaster
ERROR: Build step failed with exception
java.lang.NullPointerException: Cannot invoke method getProperty() on null object
	at org.codehaus.groovy.runtime.NullObject.invokeMethod(NullObject.java:91)
	at org.codehaus.groovy.runtime.callsite.PogoMetaClassSite.call(PogoMetaClassSite.java:47)
	at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:47)
	at org.codehaus.groovy.runtime.callsite.NullCallSite.call(NullCallSite.java:34)
	at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:47)
	at hudson.model.Job$getProperty$0.call(Unknown Source)
	at Script1$_run_closure8.doCall(Script1.groovy:273)
...
```

After applying this correction, a run with undefined jobs would look
like this:

```
Launching job: baremetal_tests_kmaster_lmaster
Failed to find job by name 'baremetal_tests_klinux-6.1.y_lmaster'
Launching job: baremetal_tests_klinux-5.15.y_lmaster
...
```

Change-Id: Idedb693816c4928a5318397a724a6096dd544f7d