Add null check to AsyncSensorManager.requestTriggerSensorImpl.

I can't find us passing a null sensor in anywhere, but the stacktrace
in the bug shows otherwise. Unfortunately, because of the async nature
of this class, the caller is not available. With this change, we will
at least get a usefull stacktrace.

Bug: 142410226
Test: manual
Change-Id: I29547965e6e302f0b873f51e11da2010d2ccaa02