PHP 8.1 Fix cookie deletion behavior

[Sdfendor]

Description (*)

As described in the corresponding issue internal function setcookie only excepts string as value parameter and PHP 8.1 emits decription error (if not ignored by OpenMage) in case null is passed.
While the prior way of using null still works (but with deprecation error) the_ correct way to delete the cookie is using an empty string as value. PHP internally changes the value to deleted which is sent in the response header. It's not necessary to pass anything else like a period that lies in the past (using negative value when calling set method of cookie model).

Fixed Issues (if relevant)

1. Fixes PHP 8.1 Issue with the way deleting cookies is done in Mage_Core_Model_Cookie #1958

Manual testing scenarios (*)

1. Log into Admin area
2. Immediately log out and observe the response header
3. Find a Set-Cookie field with adminhtml=deleted

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All automated tests passed successfully (all builds are green)

[elidrissidev]

LGTM. PHP 8.1 seems to have deprecated passing null to non-nullable parameters for all internal functions, so I guess this error may be occurring in a lot of other areas.

[Sdfendor]

@elidrissidev You are right with this assumption. #1812 does a lot of work in this direction and I'm currently working through OpenMage with PHP 8.1 myself.

[github-actions]

Unit Test Results

1 files  ±0  1 suites  ±0   0s ⏱️ ±0s
0 tests ±0  0 ✔️ ±0  0 💤 ±0  0 ❌ ±0 
7 runs  ±0  5 ✔️ ±0  2 💤 ±0  0 ❌ ±0 

Results for commit 87a6a56. ± Comparison against base commit 3486d5a.