fix: [Validation] valid_date ErrorException when the field is not sent

[kenjis]

Description
From https://forum.codeigniter.com/thread-81505-post-394652.html

DateTime::createFromFormat(): Passing null to parameter #2 ($datetime) of type string is deprecated

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