[5.4] Attempt to simplify validation with an edge case

[themsaid]

This PR attempts to solve the issue with using required_* rules while the ConvertEmptyStringToNull middleware is applied.

Given the following data & rules:

validator(
    [
        'address_one' => null,
        'address_two' => null,
    ],
    [
        'address_one' => ['nullable','required_with:address_two', 'min:6'],
        'address_two' => ['nullable','required_with:address_one', 'min:6'],
    ]
);

With this PR, using nullable won't skip the required_with validation if the value is null and will still fail, this solves the case when you accept null as a value for all rules but rules that imply the field is required.

[miklcct]

Doesn't nullable mean NULL is acceptable in addition to other valid values?

In the given example, I expect that the validation passes because both fields are nullable and really NULL. However, if both fields are empty strings or empty collections, the validation would fail.

This fix has broken my app assuming that NULL is always a valid value when nullable is specified even in presence of implicit checks, and I have to patch my validator to revert to the pre-fix behaviour.

[decadence]

@themsaid is this affecting required rule or it is only for conditional required rules?