apps/app_confbridge: Ensure 'normal' users hear message when last marked leaves

apps/app_confbridge: Ensure 'normal' users hear message when last marked leaves

When r428077 was made for ASTERISK-24522, it failed to take into account users
who are neither wait_marked nor end_marked. These users are *also* supposed to
hear the 'leader has left the conference' message. Granted, this behaviour is
a bit odd; however, that is how it used to work... and behaviour changes are
not good.

This patch ensures that if there are any 'normal' users present when the last
marked user leaves the conference, the message will still be played to them.

Note that this regression was caught by the Asterisk Test Suite's
confbridge_nominal test, which has a quirky combination of users.
........

Merged revisions 428113 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@428114 65c4cc65-6c06-0410-ace0-fbb531ad65f3