Check for null callback

The modelData callback may be nullified before actually being invoked,
e.g. when calling startRecognition() on an already running model, with
a different callback.

Fixes: 225768984
Test: Manual reproduction on the issue and then applying the fix and
      verifying that it works.
Change-Id: I089061123c8cf12eef19fdf16bb3a2c106ea2438