Fixes #71716

What?

This PR prevents empty contentSize and wideSize attributes from being saved. As a result, this prevents the unstable display of wide and full-width controls on child blocks.

Why?

In a constrained layout, such as a Group block, child blocks refer to the global ( or block instance) width setting to determine whether they can use wide or full-width controls:

<!-- wp:group {"layout":{"type":"constrained"}} -->
<div class="wp-block-group">
<!-- /wp:group -->

However, if you enter a number and then delete it, the block will change to this, even though the text control will remain empty as before:

<!-- wp:group {"layout":{"type":"constrained","contentSize":"","wideSize":""}} -->
<div class="wp-block-group"></div>
<!-- /wp:group -->

This means that the conditional statement here will no longer be met, and the child block will no longer be able to use the wide/full width options.

How?

From my understanding, empty strings should not be stored in the first place.

Testing Instructions

• Insert a Group block.
• Insert a Heading block inside that block.
• Confirm that the Heading block has both "Wide width" and "Full width" settings.
• Select the Group block and open the layout panel.
• Enter numbers in both "Content width" and "Widewidth" and delete each one.
• Select the Heading block:
  • Before: The Heading block does not have either "Wide width" or "Full width".
  • After: The Heading block has both "Wide width" and "Fullwidth" settings.