Optimization to manager events.

When potentially sending manager events, return immediately if there are no
sessions or hooks. Also, avoid locking the hooks list if it is empty.

(issue #16455)
Reported by: atis
Patches: 
      manager_hooks_trunk.patch uploaded by atis (license 242)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@243986 65c4cc65-6c06-0410-ace0-fbb531ad65f3