PHP8 fixes in the upgrader
The upgrader to Summernote 1.2 has two lines that aren't PHP8 compatible. One is a deprecation, but the other causes an error:
Applying database upgrades from extensions
[PHP Deprecation] json_decode(): Passing null to parameter #1 ($json) of type string is deprecated at /var/www/www.leonardo.info/web/sites/all/civicrm-custom/extensions/summernote/CRM/Summernote/Upgrader.php:19
In Upgrader.php line 20:
array_key_exists(): Argument #2 ($array) must be of type array, null given
This MR fixes both.