Fix no session set when replaceMWLogin is true

The session is normally set by SpecialUserlogin, when this is replaced, there
will no session setup and the login/create account will normally fail. Check
in SpecialPage if the session is already setup or setup by yourself.

Bug: 69199
Change-Id: Ib80abbcce7747104a1fa51fee52abc5af52b5f4d
(cherry picked from commit a12ac08d9620df51c91d723180507574516596b1)