[Block] Cover: Fix resize handler gets stuck on undo

[yogeshbhutkar]

What, Why, and How?

Closes #70819

This PR fixes a bug where the resize handler doesn't reset correctly after the first undo following a Cover block resize.

This occurs because the following line only checks whether minHeightUnit is set to px.

Ref:

gutenberg/packages/block-library/src/cover/edit/index.js

Line 327 in 2a278af

height: minHeightUnit === 'px' ? minHeight : 'auto',

After resizing and undoing, the unit may still be px, but minHeight becomes undefined. In this case, the resizableBox height should default to auto.

Ref. of a similar condition:

gutenberg/packages/block-library/src/cover/edit/index.js

Lines 332 to 335 in 2a278af

	const minHeightWithUnit =
	minHeight && minHeightUnit
	? `${ minHeight }${ minHeightUnit }`
	: minHeight;

Testing Instructions

1. Create a new post, insert a "Cover" block, and attach an image/video to it.
2. Resize the "Cover" block.
3. After resizing, undo the changes by pressing CMD/CTRL + Z.
4. Confirm that the resize handler gets reset properly.

Testing Instructions for Keyboard

Same.

Screenshots

Before	After

[github-actions]

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Co-authored-by: yogeshbhutkar <yogeshbhutkar@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[Mamaduka]

Thanks, @yogeshbhutkar!