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 2ae79a3)
(cherry picked from commit 688eb20)
(cherry picked from commit 484d31a)
(cherry picked from commit 8b6ae95)

Author: ldzhong

src/shared/conf-parser.c

@@ -693,6 +693,10 @@ 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;