ctdb-scripts: Avoid no-op "ctdb ptrans" call

ctdb-scripts: Avoid no-op "ctdb ptrans" call

This causes unnecessary g_lock activity and overhead.

This could be optimised in ctdb.c:control_ptrans().  However, that
makes the code more complex.  Let's only do that if we get more
potentially no-op uses.

Note no optimisation is needed in the "notify" case because there is
already an early exit if there are no items.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>