Fix possible NPE in SyncHandler.onBootCompleted()

If onBootCompleted() was called a second time, it would have throw a
NullPointerException since the queue object would have already been
set to null by the previous call. This commit just makes such an
object behave as an empty queue instead of throwing the exception.

Change-Id: I7ef82ba1e2504ceb0a2c11694d32f0588db7d0af