Check for null displayText

In my dev environment with the latest tern.java and angularjs plugins I have seen this NPE several times:
```
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 tern.eclipse.jface.text.TernBrowserInformationControlInput.<init>(TernBrowserInformationControlInput.java:43)
	at org.eclipse.angularjs.internal.ui.taginfo.HTMLAngularTagInfoHoverProcessor.getHoverInfo2(HTMLAngularTagInfoHoverProcessor.java:88)
	at org.eclipse.wst.sse.ui.internal.taginfo.BestMatchHover.getHoverInfo2(BestMatchHover.java:139)
	at org.eclipse.jface.text.TextViewerHoverManager$4.run(TextViewerHoverManager.java:166)

```

This commit adds a simple null check