make sure justifyContent exists before using it

[aristath]

Description

I started getting PHP errors in my installation today (WP trunk, Gutenberg trunk):
PHP Notice: Undefined index: justifyContent in /var/www/src/wp-content/plugins/gutenberg/lib/block-supports/layout.php on line 170

This PR simply checks that justifyContent exists before using it, in order to avoid undefined-index errors.

How has this been tested?

After the patch I no longer get errors.

Types of changes

Added an empty() check.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[youknowriad]

I think folks are working on removing that class entirely. cc @ntsekouras

[ntsekouras]

Actually on Friday @mkaz had this PR for this (now merged), but this will be removed by either reverting as I proposed or by this PR if we keep these changes.