PHP 8.1 | WP_Scripts::localize(): fix autovivification from false to array bug

This function was previously already problematic as it does not do proper input validation and the function already received tweaks related to PHP 8.0 in [50408] / Trac#52534, which also introduced the `_doing_it_wrong()` and added tests.

The short of it is:
* The function expects to receive an `array` for the `$l10n` parameter;
* ... but silently supported the parameter being passed as a `string`;
* ... and would expect PHP to gracefully handle everything else or throw appropriate warnings/errors.

In the previous fix, a `_doing_it_wrong()` was added for all non-array inputs.
The function would also cause a PHP native "Cannot use a scalar value as an array" warning (PHP < 8.0)/error (PHP 8.0) for all scalar values, except `false`.

PHP 8.1 deprecated autovivification from `false` to `array`, so now `false` starts throwing a "Automatic conversion of false to array is deprecated" deprecation notice.

By rights, the function should just throw an exception when a non-array/string input is received, but BC...

So, while I really hate doing this, the current change will maintain the previous behaviour, but will prevent both the "Cannot use a scalar value as an array" warning/error as well as the "Automatic conversion of false to array" deprecation notice for invalid inputs.

Invalid inputs _will_ still receive a `_doing_it_wrong()`, which is the only reason I find this fix even remotely acceptable.

Includes adding a test passing an empty array.
Includes adding an additional test to the data provider for a `null` input to safeguard the function will not throw a PHP 8.1 "passing null to non-nullable" notice.

Fixes:
```
3) Tests_Dependencies_Scripts::test_wp_localize_script_data_formats with data set #8 (false, '[""]')
Automatic conversion of false to array is deprecated

/var/www/src/wp-includes/class.wp-scripts.php:514
/var/www/src/wp-includes/functions.wp-scripts.php:221
/var/www/tests/phpunit/tests/dependencies/scripts.php:1447
/var/www/vendor/bin/phpunit:123
```

Refs:
* https://www.php.net/manual/en/migration81.deprecated.php#migration81.deprecated.core.autovivification-false

Author: jrfnl

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

@@ -501,12 +501,17 @@ public function localize( $handle, $object_name, $l10n ) {
 				),
 				'5.7.0'
 			);
+
+			if ( false === $l10n ) {
+				// This should really not be needed, but BC...
+				$l10n = array( $l10n );
+			}
 		}
 
 		if ( is_string( $l10n ) ) {
 			$l10n = html_entity_decode( $l10n, ENT_QUOTES, 'UTF-8' );
-		} else {
-			foreach ( (array) $l10n as $key => $value ) {
+		} elseif ( is_array( $l10n ) ) {
+			foreach ( $l10n as $key => $value ) {
 				if ( ! is_scalar( $value ) ) {
 					continue;
 				}

tests/phpunit/tests/dependencies/scripts.php

@@ -1428,17 +1428,8 @@ public function test_wp_enqueue_code_editor_when_simple_array_will_be_passed() {
 	 *
 	 * @param mixed  $l10n_data Localization data passed to wp_localize_script().
 	 * @param string $expected  Expected transformation of localization data.
-	 * @param string $warning   Optional. Whether a PHP native warning/error is expected. Default false.
 	 */
-	public function test_wp_localize_script_data_formats( $l10n_data, $expected, $warning = false ) {
-		if ( $warning ) {
-			if ( PHP_VERSION_ID < 80000 ) {
-				$this->expectWarning();
-			} else {
-				$this->expectError();
-			}
-		}
-
+	public function test_wp_localize_script_data_formats( $l10n_data, $expected ) {
 		if ( ! is_array( $l10n_data ) ) {
 			$this->setExpectedIncorrectUsage( 'WP_Scripts::localize' );
 		}
@@ -1471,14 +1462,16 @@ public function data_wp_localize_script_data_formats() {
 			array( array( 'foo' => array( 'bar' => 'foobar' ) ), '{"foo":{"bar":"foobar"}}' ),
 			array( array( 'foo' => 6.6 ), '{"foo":"6.6"}' ),
 			array( array( 'foo' => 6 ), '{"foo":"6"}' ),
+			array( array(), '[]' ),
 
 			// Unofficially supported format.
 			array( 'string', '"string"' ),
 
 			// Unsupported formats.
-			array( 1.5, '1.5', true ),
-			array( 1, '1', true ),
+			array( 1.5, '1.5' ),
+			array( 1, '1' ),
 			array( false, '[""]' ),
+			array( null, 'null' ),
 		);
 	}