Prevent initial empty state to be propagated in DisplayRepository

Depending on timings, it was possible for the `scan` to propagate the initial state first before calculating the next state based on available display ids.
In this cl the initial state is filtered out, and a log.wtf is added in the case enabledDisplays is empty (which should never happen)

Bug: 361989878
Test: DisplayRepositoryTest#displayFlow_onlyDefaultDisplayAvailable_neverEmitsEmptySet
Flag: NONE small bugfix
Change-Id: I7b7ee6883ca5b5fd95304a0a5f1168d8b5ce2e4e