Fix passing null of type string is deprecated warning

[raicabogdan]

Hello!

• Type: bug fix
• Link to issue: https://github.com/phalcon/cphalcon/issues/16649

In raising this pull request, I confirm the following:

• [x] I have read and understood the Contributing Guidelines
• [x] I have checked that another pull request for this purpose does not exist
• [ ] I wrote some tests for this PR
• [ ] I have updated the relevant CHANGELOG
• [ ] I have created a PR for the documentation about this change

Small description of change: Ensure null type deprecation warnings are avoided, happens if someone creates the same cookie twice eg:

$this->cookies->set("test", "test value");
$this->cookies->set("test", "test value");

it will cause the deprecated warning message.

Thanks