[FIX] mail : close #2250. If trying to send empty message in chatter, no other...

[FIX] mail : close #2250. If trying to send empty message in chatter, no other messages can be send wihtout refreshing page. Moving the flag_post allow sending a new message only when the previous one is sucessufully sent. If case of empty message, nothing will happen, and the chatter will not be blocked.