Fix error in opcode result processing

LUXI queries are processed without callbacks (see
server.masterd.ClientOps._Query). With commit 07923a3c the logic for
checking an opcode's result for jobs to submit was changed and
subsequently raised an exception (“'NoneType' object has no attribute
'SubmitManyJobs'”) in such a case. Before said commit the exception
would also have been raised if an opcode used by a query submitted jobs.

This patch changes the logic to only resolve the method if callbacks are
defined and to use an exception-raising implementation otherwise.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>