[BUGFIX] Handle `null` values in input transformation in AJAX requests

The module `@typo3/core/ajax/input-transformer` is used to automatically
prepare payload to be sent via AJAX. One sub-routine converts nested
objects into a flat list recursively. However, the check for objects was
not sufficient as, in JavaScript, `null` is also of type `object`. This
patch hardens the check to consider non-null values only.

Resolves: #102256
Releases: main, 12.4
Change-Id: I6cf902f6a4fa045a4f8304059607f8a359a97186
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81556


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Francois Suter <sutfra@gmail.com>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Francois Suter <sutfra@gmail.com>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>