Fix statsd NPE on setPullAtomCallback

Suspected root cause: if a process crashes right after calling
setPullAtomCallback, it's possible that oneway binder calls can queue
but do not execute before the process crashes. Then, when the process
crashes, nothing has a strong reference to the IPullAtomCallback, so
it gets deallocated. Then, when the oneway call actually executes, the
callback is null. This is being followed up in b/155793159

Regardless, statsd should handle null input properly.

Test: GTS test in ag/11348719 now passes
Test: atest GtsStatsdHostTestCases
Bug: 153822941
Change-Id: Ic6d415e10eca8d133290de80cb61e1634590ca6a