l2pop: get_agent_ports: Don't yield (None, {})

There's no point to yield None lvm with empty entries.

Closes-Bug: #1355759
Related: blueprint ofagent-l2pop
Change-Id: I42aeb48a062d35038116978d70c8dac4139a5583
Merge "l2pop: get_agent_ports: Don't yield (None, {})"