Never fire the callback in sirius.ready twice

Sometimes the DOMContentLoaded event is fired after document.readyState already changed to 'interactive'. In this case, the callback is actually called twice with our approach.

This commit ensures that the callback is only called once.

Fixes: SE-14083

src/main/resources/default/assets/common/core.js.pasta

@@ -124,10 +124,11 @@ sirius.isFilled = function (value) {
  * @param callback the callback to execute once the DOM is completely ready
  */
 sirius.ready = function (callback) {
-    // Add as listener in case DOM is loading...
-    document.addEventListener("DOMContentLoaded", callback);
-    // Call manually is we're late...
-    if (document.readyState === "interactive" || document.readyState === "complete") {
+    if (document.readyState === "loading") {
+        // Add as listener in case DOM is loading...
+        document.addEventListener("DOMContentLoaded", callback);
+    } else {
+        // Call manually if we're late...
         callback();
     }
 }