patch 9.0.1504: no error when calling remote_startserver("")

Problem:    No error when calling remote_startserver() with an empty string.
Solution:   Give an error for an empty string. (Hirohito Higashi,
            closes #12327)
diff --git a/src/testdir/test_clientserver.vim b/src/testdir/test_clientserver.vim
index 64c9ab8..53947f4 100644
--- a/src/testdir/test_clientserver.vim
+++ b/src/testdir/test_clientserver.vim
@@ -182,7 +182,8 @@
     endif
   endtry
 
-  call assert_fails('call remote_startserver([])', 'E730:')
+  call assert_fails('call remote_startserver("")', 'E1175:')
+  call assert_fails('call remote_startserver([])', 'E1174:')
   call assert_fails("let x = remote_peek([])", 'E730:')
   call assert_fails("let x = remote_read('vim10')",
         \ has('unix') ? ['E573:.*vim10'] : 'E277:')