I have been unable to use Google Talk with Empathy since I updated to 10.10. I have this problem on three computers, one which is updated and two that were fresh installs (4 months apart, so a different image).
The error I get is:
"Disconected - No reason specified"
I've found empathy/pidgin to have problems with gtalk in general, so you might be better off using gajim to connect to gtalk. It doesn't support video I think, but it is more stable in terms of the connection.
You have to type the full email address including the domain.
Or you may use this handy trick to do all you can do with google talk, audio/video chatting is also supported and the best part is this gtalk is developed by google itself http://tutafuta.com/2011/05/25/how-to-use-gtalk-on-linux-or-mac/