[Style Engine]: Defensive guarding for when style does not define the 'individual' property

[hellofromtonya]

Closes #43121

What?

This PR adds defensive guarding to protect against when the style $property_keys do not define the 'individual' property.

Why?

Prevents a PHP Notice "Undefined index: individual" from being thrown in WP_Style_Engine::get_css_declarations() when a style is not an 'individual'.

How?

It bails out early by not iterating through the array of $style_value. Why? None of that style is individual.

Testing

• Step 1: Turn on WP_DEBUG and run the PHPUnit tests. How?

Add the following code about loading Composer deps in the phpunit/bootstrap.php file:

// Test with WordPress debug mode (default).
if ( ! defined( 'WP_DEBUG' ) ) {
	define( 'WP_DEBUG', true );
}

• Step 2: Locally run the PHPUnit tests before applying this patch. Notice the following PHP Notice:

WP_Style_Engine_Test::test_generate_get_styles with data set "invalid_classnames_options" (array(array(array('friends'), array('tasty'))), array(), array())
Undefined index: individual

/var/www/html/wp-content/plugins/gutenberg/packages/style-engine/class-wp-style-engine.php:422
/var/www/html/wp-content/plugins/gutenberg/packages/style-engine/class-wp-style-engine.php:338
/var/www/html/wp-content/plugins/gutenberg/packages/style-engine/class-wp-style-engine.php:562
/var/www/html/wp-content/plugins/gutenberg/packages/style-engine/phpunit/class-wp-style-engine-test.php:37
phpvfscomposer:///var/www/html/wp-content/plugins/gutenberg/vendor/phpunit/phpunit/phpunit:[52](https://github.com/WordPress/gutenberg/runs/7756878353?check_suite_focus=true#step:7:53)

• Step 3: Apply this patch.

• Step 4: Run the PHPUnit tests again. The above PHP Notice is resolved.

Notes:

PR #41302 turns on debug mode for testing PHP notices and deprecations.

[ramonjd]

Tested with WP_DEBUG set to true

In trunk I see the warning:

14) WP_Style_Engine_Test::test_generate_get_styles with data set "invalid_classnames_options" (array(array(array('friends'), array('tasty'))), array(), array())
Undefined index: individual

/var/www/html/wp-content/plugins/gutenberg/packages/style-engine/class-wp-style-engine.php:423
/var/www/html/wp-content/plugins/gutenberg/packages/style-engine/class-wp-style-engine.php:334
/var/www/html/wp-content/plugins/gutenberg/packages/style-engine/class-wp-style-engine.php:563
/var/www/html/wp-content/plugins/gutenberg/packages/style-engine/phpunit/class-wp-style-engine-test.php:37
phpvfscomposer:///var/www/html/wp-content/plugins/gutenberg/vendor/phpunit/phpunit/phpunit:52

With the new check, it's gone 🪄

Thanks for catching @hellofromtonya

[andrewserong]

Cherry-picked into the 13.9.0 RC 3 (https://github.com/WordPress/gutenberg/releases/tag/v13.9.0-rc.3)