Device control classes are output as visible text when core/post-featured-image has no featured image #133

When using Content Control's device visibility rules on a Gutenberg core/post-featured-image block, the cc-hide-on-* classes are output as visible text if the post does not have a featured image.

Environment
WordPress: Gutenberg / native Query Loop
Content Control: 2.6.5
Block: core/post-featured-image
Content Control: Device Rules (hideOn)
Theme: Blocksy
Steps to reproduce
Create a post and add a featured image.
Add a core/post-featured-image block to a Query Loop.
Configure Content Control device rules, for example:
Desktop image: hide on mobile
Mobile image: hide on tablet and desktop
Create another post using the same Query Loop, but without a featured image.
View the Query Loop.

When a featured image exists, the classes are added correctly to the generated

When no featured image exists, core/post-featured-image returns no HTML output. Content Control nevertheless tries to add the device classes.

The result is visible text such as:

class="cc-hide-on-mobile"

or:

class="cc-hide-on-tablet cc-hide-on-desktop"

being displayed on the page.

Expected behavior

If the block produces no HTML output, Content Control should simply leave the output unchanged.

In other words, if $block_content is empty and there is no HTML element to which the classes can be added, the function should return $block_content without modifying it.

Possible cause

In classes/Controllers/Frontend/Blocks.php, render_block() searches for the first HTML element:

$html_element_matches = [];
preg_match(
'/<[^>]+>/',
$block_content,
$html_element_matches,
PREG_OFFSET_CAPTURE
);

$first_element = $html_element_matches[0][0];

For core/post-featured-image without a featured image, $block_content is empty, so there is no matching HTML element.

A possible safeguard would be:

$html_element_matches = [];

preg_match(
'/<[^>]+>/',
$block_content,
$html_element_matches,
PREG_OFFSET_CAPTURE
);

if ( empty( $html_element_matches ) ) {
return $block_content;
}

$first_element = $html_element_matches[0][0];

This would prevent Content Control from attempting to inject the classes when there is no HTML element available.

The device-control functionality itself works correctly when the featured image exists; the issue appears to be specifically related to blocks that render an empty string.