fix(form-core): don't remove event listeners if _focusableNode was not set

This fix is similar to the previous commit.

Under very specific circumstances on a very complicated application logic, a file input field (that
extends from FocusMixin) gets created and immediately destroyed. This unfortunately causes an
error, because the `disconnectedCallback()` gets called to remove the event listeners for a
component that hasn't been fully set up.

> Uncaught TypeError: this._focusableNode is null

packages/ui/components/form-core/src/FocusMixin.js

@@ -198,6 +198,7 @@ const FocusMixinImplementation = superclass =>
      * @private
      */
     __teardownEventsForFocusMixin() {
+      if (!this._focusableNode) return;
       this._focusableNode?.removeEventListener(
         'focus',
         /** @type {EventListenerOrEventListenerObject} */ (this.__redispatchFocus),