[PiP on Desktop] Don't start Transition to update inset if wct is empty.

This is a follow-up to I3e17a0ac4c030fae0f3f0003218e1342334465a2, which
caused a performance regression. This CL updates the previous CL to only
start a Transition if the wct is not empty.

Bug: 411269706
Test: atest DesktopModeWindowDecorationTests
Flag: com.android.window.flags.enable_desktop_windowing_pip
Change-Id: I354138cc55dff49b470cd80773e5a959b37fb4cb