udev: skipping empty udev rules file while collecting the stats

To keep align with the logic used in udev_rules_parse_file(), we also
should skip the empty udev rules file while collecting the stats during
manager reload. Otherwise all udev rules files will be parsed again whenever
reloading udev manager with an empty udev rules file. It's time consuming
and the following uevents will fail with timeout.

(cherry picked from commit 2ae79a31b7c7947e2c16e18eb85ac5607ebc40b6)
(cherry picked from commit 688eb20fdb9c4bcc6d205323f9cec119d6273169)

src/shared/conf-parser.c

@@ -763,6 +763,10 @@ int config_get_stats_by_path(
                         return -errno;
                 }
 
+                /* Skipping an empty file. */
+                if (null_or_empty(&st))
+                        continue;
+
                 r = hashmap_put_stats_by_path(&stats_by_path, *f, &st);
                 if (r < 0)
                         return r;