Widgets: Use isset() in WP_Widget:: display_callback() to support ArrayIterator and ArrayObject.

[33696] introduced support returning `ArrayIterator` and `ArrayObject` objects from `WP_Widget::get_settings()`. 

Per the PHP manual, `array_key_exists()` stopped supporting this in PHP 8.0.0 and deprecated in PHP 7.4.0.

>For backward compatibility reasons, array_key_exists() will also return true if key is a property defined within an object given as array. This behaviour is deprecated as of PHP 7.4.0, and removed as of PHP 8.0.0.

This commit uses `isset()` instead of `array_key_exists()` which is supported on all current versions of PHP.

Includes unit tests.

Ref:
* https://www.php.net/manual/en/function.array-key-exists.php#refsect1-function.array-key-exists-notes

Follow-up to [32602], [33696].

Props dlh, hellofromTonya, jrf, sergeybiryukov.
Fixes #52728.

git-svn-id: https://develop.svn.wordpress.org/trunk@52173 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-widget.php

@@ -365,7 +365,7 @@ class WP_Widget {
 		$this->_set( $widget_args['number'] );
 		$instances = $this->get_settings();
 
-		if ( array_key_exists( $this->number, $instances ) ) {
+		if ( isset( $instances[ $this->number ] ) ) {
 			$instance = $instances[ $this->number ];
 
 			/**