Fix 0011394: Lost password email not sent when user language is invalid

When the language specified in the preferences for a user does not exist
or is otherwise invalid (for example: when updating from previous
versions of MantisBT), the lost password email is not sent to the user.

This is probably because of lang_push( user_pref_get_language( user_id )
) where user_pref_get_language can return false and lang_push expects a
string (or null).