QQuickSpriteEngine do not use OpenGL without checking for GLContext

In the case that QQuickSpriteEngine::assembledImage() is called when
there is no current OpenGL context, return a null QImage instead of
crashing.  This is needed for the Qt Quick 2d Renderer case.

Change-Id: I75b9b1f31f05cc0800293435d660e498fecc4d20
Reviewed-by: aavit <eirik.aavitsland@digia.com>
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>