Remove erroneous precondition for null surface test

When DisplayContent is mirroring a VirtualDisplay with no surface,
it is expected that querying the surface size returns null.

Test mistakenly included an override for this method, leading to
the test case asking SurfaceFlinger to mirror a null surface.

Bug: 200017844
Test: atest WmTests:DisplayContentTests#testVirtualDisplayContent_withoutSurface --iterations 20
Change-Id: Ibaef4b85b99b6a3829ef598e050393db1616fdee