Don't polish items if the screen is 0

The current code leads to an assertion during qapp
destruction time, as the screen gets set to 0, which
triggers a force polish that in turns accesses the qapp instance.

In addition, we don't need to do any work when the screen is 0,
as the window can't be visble in that case. Once it gets visible
again, forcePolish() will be called again.

Change-Id: I81d9a11e494b4573a56d06bca651cc6b5fa785c7
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>