app_queue: update RT members when the 1st call joins a queue with no agents

If a call enters on a queue and the members on that queue are updated in
realtime (ex: using mysql inserting a new agent) the queue members are
never refreshed and the call will stay in the queue until other event occurs.
This happens only if this is the first call of the queue and there is no
agents servicing.
This patch prevent this issue, ensuring realtime members are updated if
there is one call in the queue and no available agents

ASTERISK-25442 #close

Change-Id: If1e036d013a5c1d8b0bf60d71d48fe98694a8682