Fix invalid Spacer markup when set to fill (WordPress#51317)

* Fix invalid Spacer markup when set to fill

* Try unsetting default height in save

* Update comment

Co-authored-by: Ramon <ramonjd@users.noreply.github.com>

---------

Co-authored-by: Ramon <ramonjd@users.noreply.github.com>

packages/block-library/src/spacer/save.js

@@ -3,12 +3,17 @@
  */
 import { useBlockProps, getSpacingPresetCssVar } from '@wordpress/block-editor';
 
-export default function save( { attributes: { height, width } } ) {
+export default function save( { attributes } ) {
+	const { height, width, style } = attributes;
+	const { layout: { selfStretch } = {} } = style || {};
+	// If selfStretch is set to 'fill' or 'fit', don't set default height.
+	const finalHeight =
+		selfStretch === 'fill' || selfStretch === 'fit' ? undefined : height;
 	return (
 		<div
 			{ ...useBlockProps.save( {
 				style: {
-					height: getSpacingPresetCssVar( height ),
+					height: getSpacingPresetCssVar( finalHeight ),
 					width: getSpacingPresetCssVar( width ),
 				},
 				'aria-hidden': true,