Don't handle offline messages if the corresponding chat room has already been created.
This eliminates duplicate messages in the transcript window when the user reconnects while the chat room is open.
Fix chat room existence check and chat room creation.
Existence check should take into account the actual type of the parameter passed to the method.
There it is important to distinguish JID (EntityJid type) and bare JID (EntityBareJid type) since both are used:
bare JIDs - within one-on-one chats;
full JIDs - within private chats of group chat members.
