[test] Fix a potential NPE in test environment.

This error occurs only in test environment (JUnit). We have randomly
some null resource here, this condition will allow us to facilitate the
culprit detection. The message logged will allow us to know which
resource is not in the resourceSet.
It seems that some tests do not clean correctly the env or have not time
to do it before the other tests are launched.
If the resource is null this method returns true as if the fragment is
null.

Corresponding stack: Caused by: java.lang.NullPointerException
	at
org.eclipse.sirius.diagram.tools.internal.resource.CustomSiriusDocumentProvider.isModifiable(CustomSiriusDocumentProvider.java:351)
	at
org.eclipse.gmf.runtime.diagram.ui.resources.editor.parts.DiagramDocumentEditor.isEditable(DiagramDocumentEditor.java:341)
	at
org.eclipse.gmf.runtime.diagram.ui.resources.editor.parts.DiagramDocumentEditor.updateState(DiagramDocumentEditor.java:834)
	at
org.eclipse.gmf.runtime.diagram.ui.resources.editor.parts.DiagramDocumentEditor.sanityCheckState(DiagramDocumentEditor.java:732)
	at
org.eclipse.gmf.runtime.diagram.ui.resources.editor.parts.DiagramDocumentEditor.safelySanityCheckState(DiagramDocumentEditor.java:712)
	at
org.eclipse.gmf.runtime.diagram.ui.resources.editor.parts.DiagramDocumentEditor$ActivationListener.handleActivation(DiagramDocumentEditor.java:1183)
	at
org.eclipse.gmf.runtime.diagram.ui.resources.editor.parts.DiagramDocumentEditor$ActivationListener.access$0(DiagramDocumentEditor.java:1176)
	at
org.eclipse.gmf.runtime.diagram.ui.resources.editor.parts.DiagramDocumentEditor$ActivationListener$1.run(DiagramDocumentEditor.java:1210)
	at org.eclipse.swt.widgets.RunnableLock.run(RunnableLock.java:35)
	at
org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Synchronizer.java:135)
	... 65 more
	
Change-Id: I347556982773b129e675d86152d6e9485e3c59a2
Signed-off-by: Laurent Redor <laurent.redor@obeo.fr>