Fix NPE of NativeTombstoneManager.TombstoneWatcher

Root Cause:
"path" argument of FileObserver.onEvent(event, path) can be null,
but there is no null check in NativeTombstoneManager.TombstoneWatcher.

Solution:
Do nothing when path is null

Bug: 301410299
Test: manual (check native crash dump behavior)
Change-Id: I92ee2ddd0a844e9150ca851f0d94f60f4a20e68f