reporting: isset vs empty on RRD enable

The config.xml.sample also has a <enable/> node and saving RRD by
disabling it produces an empty <rrd/> node in the code so isset()
is still the right thing to do.

PR: https://forum.opnsense.org/index.php?topic=43641.0

src/etc/inc/plugins.inc.d/core.inc

@@ -298,7 +298,7 @@ function core_cron()
     /**
      * rrd graph collector, only schedule execution when enabled
      */
-    if (!empty($config['rrd']['enable'])) {
+    if (isset($config['rrd']['enable'])) {
         $jobs[]['autocron'] = [
             '/usr/local/bin/flock -n -E 0 -o /tmp/updaterrd.lock /usr/local/opnsense/scripts/health/updaterrd.php',
             '*'