Handle case where NN AIDL callback is null in IDevice::prepareModel*

Prior to this change, if IDevice::prepareModel* was passed a null
callback, the code would still attempt to call "notify" on the callback
to return the error to the client. This CL ensures the "notify" method
will not be invoked if the callback is null.

Bug: N/A
Test: mma
Test: presubmit
Change-Id: I4a15d02c4879a0261ec26cc0e7a47d0a4da86b8b