PHP 8.1 | WP_REST_Users_Controller::update_item(): fix passing null to non-nullable

Not all requests are accompanied by a `$request['email']`. This leads to a PHP 8.1 "passing null to non-nullable" deprecation notice when the `WP_REST_Users_Controller::update_item()` method passes a `null` email address onto `email_exists()`, which eventually reached the `WP_User::get_data_by()` method where things go wrong.

In the next condition in the code of the `WP_REST_Users_Controller::update_item()` method - `if ( $owner_id && $owner_id !== $id )` - you can see that the code already takes this into account as it will not throw a `WP_Error` if `$owner_id` is falsey.

`WP_User::get_data_by()` returns `false` for a failed field request. The other functions through which the return value is passed through, do the same.

So, by setting a default value for `$owner_id` of `false` and only checking `email_exists()` when there is an email to check, the "passing null to non-nullable" deprecation notice is bypassed without breaking BC.

Fixes a whole slew of test errors along the lines of:
```
6) WP_Test_REST_Users_Controller::test_update_item_en_US_locale
trim(): Passing null to parameter #1 ($string) of type string is deprecated

/var/www/src/wp-includes/class-wp-user.php:211
/var/www/src/wp-includes/pluggable.php:105
/var/www/src/wp-includes/user.php:1953
/var/www/src/wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php:728
/var/www/src/wp-includes/rest-api/class-wp-rest-server.php:1143
/var/www/src/wp-includes/rest-api/class-wp-rest-server.php:990
/var/www/tests/phpunit/includes/spy-rest-server.php:67
/var/www/tests/phpunit/tests/rest-api/rest-users-controller.php:1719
/var/www/vendor/bin/phpunit:123
```

Author: jrfnl

src/wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php

@@ -725,7 +725,10 @@ public function update_item( $request ) {
 			);
 		}
 
-		$owner_id = email_exists( $request['email'] );
+		$owner_id = false;
+		if ( is_string( $request['email'] ) ) {
+			$owner_id = email_exists( $request['email'] );
+		}
 
 		if ( $owner_id && $owner_id !== $id ) {
 			return new WP_Error(