What? How?

Checks for label content before rendering a wp-block-social-link-label span using ! empty( $attributes['label'] ).

Also as a double precaution, setting the label attribute to undefined instead of ''.

If there's no label, render the default in the frontend.

Why?

To avoid rendering an empty wp-block-social-link-label span on the frontend.

Also as a double precaution, setting the label attribute to undefined instead of ''.

Empty strings will pass PHP's isset check.

Testing Instructions

1. Insert social icons into post and publish.
2. Observe that a default label is rendered, e.g., <span class="wp-block-social-link-label screen-reader-text">Mail</span>
3. In the editor, edit the icons' label. Save the post. Check that the labels have been updated on the frontend.
4. Now remove the same labels in the editor.
5. Now the <span class="wp-block-social-link-label screen-reader-text"/> should contain the default label from the $services_data array

Test HTML

<!-- wp:social-links {"openInNewTab":true} -->
<ul class="wp-block-social-links"><!-- wp:social-link {"url":"http://wordpress.org","service":"wordpress","label":"WordPress"} /-->

<!-- wp:social-link {"url":"mailto:example@example.org","service":"mail"} /-->

<!-- wp:social-link {"service":"fivehundredpx"} /-->

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