Autodiscovery: bail early if allowed network list is empty (librenms#17601)

* Autodiscovery: bail early if allowed network list is empty

* Apply fixes from StyleCI

Author: Jellyfrog

includes/discovery/functions.inc.php

@@ -42,6 +42,11 @@
 function discover_new_device($hostname, $device, $method, $interface = null)
 {
     Log::debug("discovering $hostname\n");
+    if (empty(Config::get('nets'))) {
+        Log::debug("Allowed discovery network list is empty - skipping\n");
+
+        return false;
+    }
 
     if (IP::isValid($hostname)) {
         $ip = $hostname;