refactor: starter key handling in SodiumHandler

[datamweb]

Description
While I recognize that this change might be seen as a BC, it appears that it will not significantly impact many users. The improved validation of the starter key in the SodiumHandler class primarily focuses on enhancing error handling and preventing potential issues related to an empty key, which should lead to a more robust encryption process.

See : https://github.com/codeigniter4/CodeIgniter4/actions/runs/11033764630/job/30645688529?pr=9202

 ------ ----------------------------------------------------------------------- 
  Line   system/Encryption/Handlers/SodiumHandler.php                           
 ------ ----------------------------------------------------------------------- 
  66     Property CodeIgniter\Encryption\Handlers\SodiumHandler::$key (string)  
         does not accept null.                                                  
         🪪  assign.propertyType                                                
  108    Property CodeIgniter\Encryption\Handlers\SodiumHandler::$key (string)  
         does not accept null.                                                  
         🪪  assign.propertyType                                                
 ------ ----------------------------------------------------------------------- 

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[michalsn]

In fact, such changes should go to branch 4.6, and here we should fix it by typing the variable, but... these changes are unlikely to affect anyone, so we might as well do it in develop.

We will also need a changelog entry.

[kenjis]

How about this?

--- a/system/Encryption/Handlers/SodiumHandler.php
+++ b/system/Encryption/Handlers/SodiumHandler.php
@@ -26,7 +26,7 @@ class SodiumHandler extends BaseHandler
     /**
      * Starter key
      *
-     * @var string
+     * @var string|null Null is used for buffer cleanup.
      */
     protected $key = '';
 

[datamweb]

Thank you all for your help, I truly appreciate it. I didn’t mean to pull all of you into this, but your support means a lot!