Optimize empty transactions

This is a micro-optimization: if we know there are no affected paths,
we used to spin an entire transaction -- which ended up revalidating an
empty delta. Let's skip the entire circus and proceed to commit
directly.

Change-Id: Ia6250663c01348dd08610c2a44dc01743761a05c
Signed-off-by: Robert Varga <rovarga@cisco.com>