Layout: Fix blockGap output when using a falsy 0 value

[andrewserong]

What?

This fixes an issue exposed by #43466 where block gap values of 0 applied at the individual block level would not be output in server-rendering due to them using a falsy value.

Why?

Previously, custom values of 0 were being stored as 0px which is a non-falsy string. However, 0 as a string is falsy in PHP so the output was being skipped.

How?

Do a non-null check instead of a truthy check for blockGap before outputting the layout styles.

Testing Instructions

1. In a post or page, add a Group block with a couple of child blocks, and set block spacing to the 0 preset value.
2. On the site frontend, check that the 0 spacing is being applied between child elements
3. Repeat with the Social Icons block

Or, test with the following block markup

Test block markup

<!-- wp:group {"layout":{"type":"constrained"},"style":{"spacing":{"blockGap":"0"}}} -->
<div class="wp-block-group"><!-- wp:paragraph -->
<p>A paragraph</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>A paragraph</p>
<!-- /wp:paragraph --></div>
<!-- /wp:group -->

<!-- wp:group {"layout":{"type":"default"},"style":{"spacing":{"blockGap":"0"}}} -->
<div class="wp-block-group"><!-- wp:paragraph -->
<p>A paragraph</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>A paragraph</p>
<!-- /wp:paragraph --></div>
<!-- /wp:group -->

<!-- wp:social-links {"style":{"spacing":{"blockGap":{"top":"0","left":"0"}}}} -->
<ul class="wp-block-social-links"><!-- wp:social-link {"url":"wordpress.org","service":"wordpress"} /-->

<!-- wp:social-link {"url":"wordpress.org","service":"wordpress"} /-->

<!-- wp:social-link {"url":"wordpress.org","service":"wordpress"} /--></ul>
<!-- /wp:social-links -->

Screenshots or screencast

With this PR applied, the output should look like the following:

[ramonjd]

Before

After

Setting block gap to 0 for social links in global styles

Axial gap values for social links

[andrewserong]

I've just removed the Backport to Gutenberg Minor Release label as the bug that this PR fixes isn't in Gutenberg 14.0 (#43466 will be part of 14.1).