Add test for onMaximizeButtonHoverExit with null view holder

Adds a unit test to verify that calling `onMaximizeButtonHoverExit()` after the window decoration has been closed does not result in a NullPointerException.

This test simulates an edge case where a hover exit event might be received after the decoration's view holder has been destroyed and nulled out, ensuring the added null check correctly prevents a crash.


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Original Change: ag/35041164
Test: ATP tests passed http://go/forrest-run/L96900030017382731
Bug: 431235865
Flag: TEST_ONLY

Change-Id: Ia416fff098b8bf18aaf0f6d306f4b16acd9f5e4d