Code Modernization: Fix null to non-nullable deprecation in WP_Theme_JSON::get_property_value().

This commit aims to fix errors caused by incorrect usage of the `strncmp()` function inside the `WP_Theme_JSON::get_property_value()` method on PHP 8.1 and above.

Some history of the affected code:
* [50973] introduced the `WP_Theme_JSON::get_property_value()` method.
* [54162] removed the `$default` parameter from the `_wp_array_get()` call in the method.

With the latter change, the default value that is returned if the path does not exist within the array, or if `$array` or `$path` are not arrays, became `null` instead of an empty string. Since `null` would then be unintentionally passed to the `strncmp()` function further in the code, this caused ~35 errors in the test suite along the lines of:
{{{
1) Tests_Blocks_Editor::test_get_block_editor_settings_theme_json_settings
strncmp(): Passing null to parameter #1 ($string1) of type string is deprecated

/var/www/src/wp-includes/class-wp-theme-json.php:1754
/var/www/src/wp-includes/class-wp-theme-json.php:1641
/var/www/src/wp-includes/class-wp-theme-json.php:2066
/var/www/src/wp-includes/class-wp-theme-json.php:1002
/var/www/src/wp-includes/class-wp-theme-json.php:898
/var/www/src/wp-includes/global-styles-and-settings.php:140
/var/www/src/wp-includes/block-editor.php:421
/var/www/tests/phpunit/tests/blocks/editor.php:388
/var/www/vendor/bin/phpunit:123
}}}

This commit includes:
* Restoring the `$default` value for `_wp_array_get()` call.
* Adding an early return if the value is an empty string or `null`.
* Adding a dedicated unit test to ensure that the method returns a string for invalid paths or `null` values.

Follow-up to [50973], [54162].

Props antonvlasenko, jrf, imadarshakshat, SergeyBiryukov.
Fixes #56620.

git-svn-id: https://develop.svn.wordpress.org/trunk@54362 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/class-wp-theme-json.php

@@ -1709,7 +1709,12 @@ protected static function compute_style_properties( $styles, $settings = array()
 	 * @return string|array Style property value.
 	 */
 	protected static function get_property_value( $styles, $path, $theme_json = null ) {
-		$value = _wp_array_get( $styles, $path );
+		$value = _wp_array_get( $styles, $path, '' );
+
+		if ( '' === $value || null === $value ) {
+			// No need to process the value further.
+			return '';
+		}
 
 		/*
 		 * This converts references to a path to the value at that path

tests/phpunit/tests/theme/wpThemeJson.php

@@ -2996,6 +2996,54 @@ public function test_get_property_value_valid() {
 		$this->assertSame( $expected, $theme_json->get_stylesheet() );
 	}
 
+	/**
+	 * Tests that get_property_value() static method returns an empty string
+	 * if the path is invalid or the value is null.
+	 *
+	 * Also, tests that PHP 8.1 "passing null to non-nullable" deprecation notice
+	 * is not thrown when passing the value to strncmp() in the method.
+	 *
+	 * The notice that we should not see:
+	 * `Deprecated: strncmp(): Passing null to parameter #1 ($string1) of type string is deprecated`.
+	 *
+	 * @dataProvider data_get_property_value_should_return_string_for_invalid_paths_or_null_values
+	 *
+	 * @ticket 56620
+	 *
+	 * @covers WP_Theme_JSON::get_property_value
+	 *
+	 * @param array $styles An array with style definitions.
+	 * @param array $path   Path to the desired properties.
+	 *
+	 */
+	public function test_get_property_value_should_return_string_for_invalid_paths_or_null_values( $styles, $path ) {
+		$reflection_class = new ReflectionClass( WP_Theme_JSON::class );
+
+		$get_property_value_method = $reflection_class->getMethod( 'get_property_value' );
+		$get_property_value_method->setAccessible( true );
+		$result = $get_property_value_method->invoke( null, $styles, $path );
+
+		$this->assertSame( '', $result );
+	}
+
+	/**
+	 * Data provider for test_get_property_value_should_return_string_for_invalid_paths_or_null_values().
+	 *
+	 * @return array
+	 */
+	public function data_get_property_value_should_return_string_for_invalid_paths_or_null_values() {
+		return array(
+			'empty string' => array(
+				'styles' => array(),
+				'path'   => array( 'non_existent_path' ),
+			),
+			'null'         => array(
+				'styles' => array( 'some_null_value' => null ),
+				'path'   => array( 'some_null_value' ),
+			),
+		);
+	}
+
 	/**
 	 * Testing that dynamic properties in theme.json that refer to other dynamic properties in a loop
 	 * should be left untouched.
@@ -3084,7 +3132,6 @@ public function test_get_property_value_self() {
 		$this->assertSame( $expected, $theme_json->get_stylesheet() );
 	}
 
-
 	/**
 	 * @dataProvider data_get_layout_definitions
 	 *