Validator docs should clarify what happens if `$when` is null

[mehov]

Hi everyone

• https://api.cakephp.org/5.0/class-Cake.Validation.Validator.html#maxLength(),
• https://api.cakephp.org/5.0/class-Cake.Validation.Validator.html#minLength(),
• …and others:

Closure|string|null $when optional Either 'create' or 'update' or a Closure that returns true when the validation rule should be applied.

Unclear what happens if null is passed.