Hi all,
I removed these errors by running the following queries in PHPMyAdmin:
UPDATE `ps_category_lang` SET `additional_description`='' WHERE `additional_description` IS NULL;
UPDATE `ps_category_lang` SET `description`='' WHERE `description` IS NULL;
UPDATE `ps_category_lang` SET `meta_description`='' WHERE `meta_description` IS NULL;
UPDATE `ps_category_lang` SET `meta_keywords`='' WHERE `meta_keywords` IS NULL;
UPDATE `ps_category_lang` SET `meta_title`='' WHERE `meta_title` IS NULL;
Thanks to @Hlavtox here: Category issue after bulk uploading categories using PHP 8.2. After downgrading to PHP 8.1, errors persist · Issue #34535 · PrestaShop/PrestaShop · GitHub
Sharing in case someone here has similar troubles with Prestashop on Yunohost.