Fix array key warning in block supports layout

[mkaz]

Description

During testing I noticed the following warning in PHP logs:

PHP Warning:  Undefined array key "justifyContent" in /home/mkaz/src/wp/gutenberg/lib/block-supports/layout.php on line 170, referer: http://localhost:8003/wp-admin/themes.php?page=gutenberg-edit-site&postId=twentytwentytwo%2F%2Fhome&postType=wp_template

This PR adds an isset() check for the justifyContent value to suppress the warning.

How has this been tested?

1. Using twentytwentytwo with no changes
2. Enable debug logging / at least warning level
3. Open site in site editor and confirm warning
4. Apply patch, confirm warning goes away
5. Modfiy a block using justifyContent confirm works

Types of changes

Switches a array check to isset()

[mkaz]

FYI - Looking at WP 5.9 and it looks like this was already addressed when moved to core. So no backport necessary.

https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-includes/block-supports/layout.php#L166

[ntsekouras]

Thanks @mkaz !