fix: check contents of $matches[1] in gutenberg_get_block_style_variation_name_from_class()

[justlevine]

What?

This PR fixes an issue in gutenberg_get_block_style_variation_name_from_class() where an array would be returned even if the variation name was not matched in the CSS class string.

This is due to the fact that preg_match_all() will always set $matches[1], and instead uses an empty string if no match is found.

Why?

While this issue was surfaced by PHPStan (via #66693) it can be remediated independently as part of #66598

How?

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

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Co-authored-by: justlevine <[email protected]>

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