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)

src/version.c

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