[FIXED JENKINS-40545] Diagnose NPE in DescriptorVisibilityFilter.apply

Before:
Jun 15, 2017 8:29:06 AM hudson.ExpressionFactory2$JexlExpression evaluate
WARNING: Caught exception evaluating: h.filterDescriptors(it, no_descriptors_here).size() in /jenkins/jenkins40545/. Reason: java.lang.NullPointerException
java.lang.NullPointerException
	at hudson.model.DescriptorVisibilityFilter.apply(DescriptorVisibilityFilter.java:72)
	at hudson.Functions.filterDescriptors(Functions.java:1863)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.apache.commons.jexl.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:258)
	...

After:
Jun 15, 2017 12:10:43 PM hudson.ExpressionFactory2$JexlExpression evaluate
WARNING: Caught exception evaluating: h.filterDescriptors(it, no_descriptors_here).size() in /jenkins/jenkins40545/. Reason: java.lang.NullPointerException: Descriptor list is null for context 'class hudson.model.DescriptorVisibilityFilterTest$Jenkins40545' in thread 'Handling GET /jenkins/jenkins40545/ from 127.0.0.1 : Jetty Thread Pool DescriptorVisibilityFilterTest/Jenkins40545/index.jelly'
java.lang.NullPointerException: Descriptor list is null for context 'class hudson.model.DescriptorVisibilityFilterTest$Jenkins40545' in thread 'Handling GET /jenkins/jenkins40545/ from 127.0.0.1 : Jetty Thread Pool DescriptorVisibilityFilterTest/Jenkins40545/index.jelly'
	at hudson.model.DescriptorVisibilityFilter.apply(DescriptorVisibilityFilter.java:73)
	at hudson.Functions.filterDescriptors(Functions.java:1863)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.apache.commons.jexl.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:258)
	...