Camera: Fix null pointer exception in onResultReceived

It's possible that a pending request is removed from
CaptureCallbackMap before its result is received.

In that case, the callback holder will be null. And we should
not dereference the null pointer.

A minor code cleanup is also made.

Test: Camera CTS
Bug: 295066682
Change-Id: I70b04ac4bb4bd4c5786677fd2117aa15eafafef6