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)

runtime/doc/builtin.txt

@@ -7312,8 +7312,9 @@ remote_send({server}, {string} [, {idvar}])
 <
 					*remote_startserver()* *E941* *E942*
 remote_startserver({name})
-		Become the server {name}.  This fails if already running as a
-		server, when |v:servername| is not empty.
+		Become the server {name}.  {name} must be a non-empty string.
+		This fails if already running as a server, when |v:servername|
+		is not empty.
 
 		Can also be used as a |method|: >
 			ServerName()->remote_startserver()

src/clientserver.c

@@ -968,25 +968,23 @@ f_remote_send(typval_T *argvars UNUSED, typval_T *rettv)
 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

src/testdir/test_clientserver.vim

@@ -182,7 +182,8 @@ func Test_client_server()
     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:')

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1504,
 /**/
     1503,
 /**/