Validations do not handle nil values

[grey280]

Describe the bug

validations.add("foo", as: String?.self, is: .nil || !.nil) throws a validation error when trying to validate a nil value.

To Reproduce

Steps to reproduce the behavior:

1. Create a Content class/struct and add a validation for a String? property using is: .nil || !.nil as the validation requirement
2. Create an endpoint that validates your new class/struct.
3. Send a request to the endpoint that either explicitly or implicitly has nil as the value of the String? property.
4. Observe the (incorrect) validation error thrown.

Expected behavior

Since the expectation is a tautology, it should always pass validation.

Environment

• Vapor Framework version: 4.76.3
• Vapor Toolbox version: 18.7.1
• OS version: macOS 13.4

Additional context

I've simplified this case down, as I originally had it doing a sane validation, but the tautological version is a better demonstration of the issue. (I was originally trying a is: .nil || .empty || .pattern(bar) check, and found it to always fail when given a nil even though that was an explicitly-allowed value.)

[0xTim]

I think this is a quirk of the validation library and our docs. If you want to verify something is either nil or matches a validator you can pass a required flag to the validation. Does that work for you?

[grey280]

Indeed that does, thanks Tim! Another read-through of the docs, I do see that it mentions the required flag, but it may be worth specifically calling out the use of that for optional types, as: String.self ... required: false rather than as: String?.self?

[0xTim]

Yeah the docs could be a bit better - if you fancy a PR then great!