timesync: actually disables built-in fallback NTP servers when an empty string is specified

Follow-up for 3745770ae4.
Fixes #40621.

src/timesync/timesyncd-conf.c

@@ -86,6 +86,11 @@ int config_parse_servers(
 
         if (isempty(rvalue)) {
                 manager_flush_server_names(m, ltype);
+
+                /* FallbackNTP= with an empty string disables the built-in fallback servers. */
+                if (ltype == SERVER_FALLBACK)
+                        m->fallback_set = true;
+
                 return 0;
         }