Fix the fallback for an empty list of highlighters

See https://www.woltlab.com/community/thread/314320/

Author: dtdesign

wcfsetup/install/files/lib/system/bbcode/BBCodeHandler.class.php

@@ -189,7 +189,9 @@ public function getHighlighters()
      */
     public function getCodeBlockLanguages(): array
     {
-        $highlighters = \explode("\n", StringUtil::unifyNewlines(\MESSAGE_PUBLIC_HIGHLIGHTERS));
+        $highlighters = \array_filter(
+            \explode("\n", StringUtil::unifyNewlines(\MESSAGE_PUBLIC_HIGHLIGHTERS))
+        );
         if ($highlighters !== []) {
             return $highlighters;
         }