RESTRICT AUTOMERGE: Don't assert not null for return value of setPropertyAndWaitForChange

Additionally, do not verify set if property not available. This is inline with ATS CPMTest which also checks for property availability before trying to set and confirm and allows for valid error codes or exceptions.

Bug: 397076962
Change-Id: I0a5e5bf9203d309a40ad44c4b800ca25af586e3d
Test: atest CtsCarTestCases:CarPropertyManagerTest