Do not send call event if handover call has no ConnectionService bound.

If the handover TO ConnectionService returns a null Connection, failing
the handover, Telecom will still try to report to the "to" Connection
that the handover failed, when it is in reality not possible to report
that.

Adding a null CS check when sending this CallEvent.

Test: Manual
Bug: 37896563
Change-Id: I4e041dade46de173247ea6110b9da0e356af606b