Don't SIGSEGV if no transport is used.
    
    One of previous commits (74b1b62510d939) allowed us to connect to
    localhost directly if ssh transport was used. However, if there's
    not transport, we SIGSEGV'ed as g_str_equal doesn't like NULL as
    one of arguments. Change this to g_strcmp0 which does the same
    service but is more friendly to NULL arguments.