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/clientserver.c b/src/clientserver.c
index dfbb8a8..400a4ad 100644
--- a/src/clientserver.c
+++ b/src/clientserver.c
@@ -968,25 +968,23 @@
 f_remote_startserver(typval_T *argvars UNUSED, typval_T *rettv UNUSED)
 {
 #ifdef FEAT_CLIENTSERVER
-    char_u	*server;
-
-    if (in_vim9script() && check_for_string_arg(argvars, 0) == FAIL)
+    if (check_for_nonempty_string_arg(argvars, 0) == FAIL)
 	return;
 
-    server = tv_get_string_chk(&argvars[0]);
-    if (server == NULL)
-	return;		// type error; errmsg already given
     if (serverName != NULL)
-	emsg(_(e_already_started_server));
-    else
     {
-# ifdef FEAT_X11
-	if (check_connection() == OK)
-	    serverRegisterName(X_DISPLAY, server);
-# else
-	serverSetName(server);
-# endif
+	emsg(_(e_already_started_server));
+	return;
     }
+
+    char_u *server = tv_get_string_chk(&argvars[0]);
+# ifdef FEAT_X11
+    if (check_connection() == OK)
+	serverRegisterName(X_DISPLAY, server);
+# else
+    serverSetName(server);
+# endif
+
 #else
     emsg(_(e_clientserver_feature_not_available));
 #endif
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:')
diff --git a/src/version.c b/src/version.c
index 338399c..2a99c3b 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1504,
+/**/
     1503,
 /**/
     1502,