[STORM-3764] Fix NPE in SchedulingSearcherState.backtrack()

[bipinprasad]

What is the purpose of the change

While backtracking in scheduling code, Ackers are skipped. However, under certain circumstances a null WorkerSlot is passed as parameter to SchedulingSearcherState.backtrack(). This null key is used to retrieve an entry from Map. The returned null map value causes NPE

How was the change tested

new unit test TestGenericResourceAwareStrategy.testScheduleLeftOverAckers()