nothos,

You're correct. It's a bug. The change was made in the interest of bringing the Admin back in line with the non-admin code. But it's a bit premature to do that in v155 without also changing a dozen other product-edit-related files, all of which are often also altered by plugins.
I've reviewed older notes about why this wasn't done to the admin initially, and this is why.
I'm a bit embarrassed that the testing didn't bear this out, but it turns out the testing server had a relaxed MySQL mode enabled for some other tests and hadn't been reset properly, so this got missed.
Sigh. Happy new year!

The quickest fix is to revert the /admin/includes/functions/database.php file to what was in v155b, by changing those 2 'NULL' back to 'null' in zen_db_perform(), lines 26 and 42.

Code:
          case 'NULL':
becomes

Code:
          case 'null':