Don't make a no-op DB call

drivers_uuid can be empty, but we are still doing a DB call. This is causing
a sqlalchemy SAWarning.

SAWarning: The IN-predicate on "instances.uuid" was invoked with an
empty sequence. This results in a contradiction, which nonetheless can
be expensive to evaluate.  Consider alternative strategies for improved
performance.

Change-Id: Ib8c9b85e84800c9e2ddcdf204851f1b51101926c
Closes-Bug: #1383617
Merge "Don't make a no-op DB call"