Check handler before trying to wait for looper

Problem: Tests that tried to put the state machine in a disconnected
state would run into a race condition where the the test logic was
racing to grab the looper object to wait on it before the class under
test cleaned up the handler via the normal state machine quit logic. If
the class under test cleaned up quickly, the test would fail with an NPE
when it made a getHandler() call.

Solution: Null check the handler. If its null, then we've got no waiting
to do. We can assert the state after to make sure we're in the state
we're supposed to be in.

Bug: 386715308
Flag: EXEMPT, test only change
Test: atest com.android.bluetooth.mapclient.MapClientStateMachineTest
Test: atest com.android.bluetooth.pbapclient.PbapClientServiceTest
Change-Id: Iaef6ec1c7eb6f0658c5470e105b9fd18bd4c2009