Query GdkWindow reference only if available

On startup, the Geany main window doesn't have a GdkWindow yet
(probably because it is not yet mapped). This causes many
'gdkdrawable-win32.c:2013 drawable is not a pixmap or window'
warnings when resolving shortcuts on Windows.
Since we pass the SLR_NO_UI to the Windows API, we probably
don't need the parent hWnd reference at all.