[BUGFIX] Properly escalate if a form HMAC fails to decode

If a HMAC cannot be `json_decode`d (possibly due to old
saved pages or other outdated content), now a proper exception
is thrown instead of issuing a PHP warning.

Resolves: #97337
Releases: main, 12.4, 11.5
Change-Id: I12f5633a85508bf4099d72e474c24b5a0100498c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81719


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>