Protect against NPE

In my dev environment with the latest eclipse-wtp-search plugins I'm seeing this NPE often:
```
org.eclipse.core.runtime.AssertionFailedException: null argument:
	at org.eclipse.core.runtime.Assert.isNotNull(Assert.java:85)
	at org.eclipse.core.runtime.Assert.isNotNull(Assert.java:73)
	at org.eclipse.jdt.internal.ui.javaeditor.EditorUtility.getEditorInputJavaElement(EditorUtility.java:442)
	at org.eclipse.wst.xml.search.editor.java.hover.Java2XHover.getHoverInfo(Java2XHover.java:41)
	at org.eclipse.jdt.internal.ui.text.java.hover.BestMatchHover.getHoverInfo2(BestMatchHover.java:169)
	at org.eclipse.jdt.internal.ui.text.java.hover.BestMatchHover.getHoverInfo2(BestMatchHover.java:129)
	at org.eclipse.jdt.internal.ui.text.java.hover.JavaEditorTextHoverProxy.getHoverInfo2(JavaEditorTextHoverProxy.java:85)
	at org.eclipse.jface.text.TextViewerHoverManager$4.run(TextViewerHoverManager.java:166)
```

This commit adds a simple null check