lb-agent: fix get_interfaces_on_bridge returning None

Change get_interfaces_on_bridge() to return [] instead of None
if the bridge is not found, as callers are expecting an iterator.

Change-Id: I359a2a0cf932c049f6891d4ba3eb8b9b23ab360d
Related-Bug: 1284311
Merge "lb-agent: fix get_interfaces_on_bridge returning None"