picture and img-srcset have no src attribute in the database, making the front end optimization crash

[MathieuLamiot]

Context
Following #6694 there is still a regression on develop for picture and img-srcset LCP/ATF.
The issue is that they can be saved in DB without the src attribute, creating a fatal:

[05-Jun-2024 17:12:05 UTC] PHP Warning:  Undefined property: stdClass::$src in /var/www/html/mathieu.e2e.rocketlabsqa.ovh/wp-content/plugins/wp-rocket/inc/Engine/Media/AboveTheFold/Frontend/Controller.php on line 155
[05-Jun-2024 17:12:05 UTC] PHP Fatal error:  Uncaught TypeError: preg_quote(): Argument #1 ($str) must be of type string, null given in /var/www/html/mathieu.e2e.rocketlabsqa.ovh/wp-content/plugins/wp-rocket/inc/Engine/Media/AboveTheFold/Frontend/Controller.php:155
Stack trace:
#0 /var/www/html/mathieu.e2e.rocketlabsqa.ovh/wp-content/plugins/wp-rocket/inc/Engine/Media/AboveTheFold/Frontend/Controller.php(155): preg_quote()
#1 /var/www/html/mathieu.e2e.rocketlabsqa.ovh/wp-content/plugins/wp-rocket/inc/Engine/Media/AboveTheFold/Frontend/Controller.php(119): WP_Rocket\Engine\Media\AboveTheFold\Frontend\Controller->set_fetchpriority()
#2 /var/www/html/mathieu.e2e.rocketlabsqa.ovh/wp-content/plugins/wp-rocket/inc/Engine/Media/AboveTheFold/Frontend/Controller.php(86): WP_Rocket\Engine\Media\AboveTheFold\Frontend\Controller->preload_lcp()
#3 /var/www/html/mathieu.e2e.rocketlabsqa.ovh/wp-content/plugins/wp-rocket/inc/Engine/Media/AboveTheFold/Frontend/Subscriber.php(46): WP_Rocket\Engine\Media\AboveTheFold\Frontend\Controller->lcp()
#4 /var/www/html/mathieu.e2e.rocketlabsqa.ovh/wp-includes/class-wp-hook.php(324): WP_Rocket\Engine\Media\AboveTheFold\Frontend\Subscriber->lcp()
#5 /var/www/html/mathieu.e2e.rocketlabsqa.ovh/wp-includes/plugin.php(205): WP_Hook->apply_filters()
#6 /var/www/html/mathieu.e2e.rocketlabsqa.ovh/wp-content/plugins/wp-rocket/inc/Engine/Optimization/Buffer/Optimization.php(100): apply_filters()
#7 [internal function]: WP_Rocket\Engine\Optimization\Buffer\Optimization->maybe_process_buffer()
#8 /var/www/html/mathieu.e2e.rocketlabsqa.ovh/wp-includes/functions.php(5373): ob_end_flush()
#9 /var/www/html/mathieu.e2e.rocketlabsqa.ovh/wp-includes/class-wp-hook.php(324): wp_ob_end_flush_all()
#10 /var/www/html/mathieu.e2e.rocketlabsqa.ovh/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters()
#11 /var/www/html/mathieu.e2e.rocketlabsqa.ovh/wp-includes/plugin.php(517): WP_Hook->do_action()
#12 /var/www/html/mathieu.e2e.rocketlabsqa.ovh/wp-includes/load.php(1260): do_action()
#13 [internal function]: shutdown_action_hook()
#14 {main}
  thrown in /var/www/html/mathieu.e2e.rocketlabsqa.ovh/wp-content/plugins/wp-rocket/inc/Engine/Media/AboveTheFold/Frontend/Controller.php on line 155

This is reproducible on this template: http://mathieu.e2e.rocketlabsqa.ovh/lcp_no_dimensions_picture
On 3.16, we have in the DB for Desktop LCP on this page:

{"type":"picture","src":"","sources":[{"srcset":"http:\/\/mathieu.e2e.rocketlabsqa.ovh\/wp-content\/rocket-test-data\/images\/lcp\/testavif.avif","media":"(min-width: 700px)"},{"srcset":"http:\/\/mathieu.e2e.rocketlabsqa.ovh\/wp-content\/rocket-test-data\/images\/lcp\/testPng.png","media":"(min-width: 450px)"}]}
On develop, the src attribute is missing.

Expected behavior
src attribute must always be set for an LCP/ATF object in the DB.
No Fatal related to missing $lcp->src must occur.

Acceptance Criteria

• LCP/ATF optimization on the template http://mathieu.e2e.rocketlabsqa.ovh/lcp_no_dimensions_picture must be applied without Fatal error

[Mai-Saad]

Fixing this at e2e shall capture this kind of error wp-media/wp-rocket-e2e#98