Do not clear realname when updating user with LDAP

When $g_use_ldap_realname = ON, wrong logic in method
UserUpdateCommand::validate() caused 'True' to be passed as username
parameter to ldap_realname_from_username(), which returned null instead
of the expected real name, as the user '1' cannot be found in the LDAP
directory (an incorrect realname would have been returned, if this user existed).

Fixes 0034566