Fix user destination parsing in simp messaging

This commit fixes the parsing of message destinations such as
"/user/anna/queue/foo", reverting a regression introduced by SPR-11506,
which worked well with @SendToUser use cases but caused issues for
messages sent to other users.

Issue: SPR-12444
Could this line be removed since the user is not used? It seems like setting the user to null was not necessary even in the old version.