The exception below is printed to standard error every time a generic
editor diff contribution is used in compare editor for target files. I
assume it never worked as designed with TargetPlatformFoldingReconciler,
so just add a NPE guard to prevent unhandled exceptions / printouts.

```
Exception in thread
"org.eclipse.pde.internal.genericeditor.target.extension.reconciler.folding.TargetPlatformFoldingReconciler"
java.lang.NullPointerException: Cannot invoke
"org.eclipse.jface.text.source.projection.ProjectionViewer.getProjectionAnnotationModel()"
because "this.projectionViewer" is null
	at org.eclipse.pde.internal.genericeditor.target.extension.reconciler.folding.FoldingReconcilerStrategy.initialReconcile(FoldingReconcilerStrategy.java:63)
	at org.eclipse.jface.text.reconciler.Reconciler.initialProcess(Reconciler.java:223)
	at org.eclipse.jface.text.reconciler.AbstractReconciler$BackgroundThread.run(AbstractReconciler.java:177)
```

See https://github.com/eclipse-platform/eclipse.platform.ui/issues/1798