[6.x] Fix nullable values for required_if

[driesvints]

required_if, required_unless, exclude_if, exclude_unless (referred to below as "these rules").

This PR fixes several things:

1. A regression introduced in [6.x] Fix required_if boolean validation #36969 (comment): we now will take into account nullable values the same way as we do forboolean values. This will make sure that if someone adds null to these rules, it'll get properly handled for the targetted key's value.
2. These rules will now automatically pass when the targeted key isn't present in the data. That's the if (! Arr::has($this->data, $parameters[0])) { part that was added to each of them. This was already expected I believe but should now fail early.
3. I've added an if (is_null($value)) { to FormatsMessages so that validation messages can display things like The baz field is required when foo is null.. This might be a bigger breaking change so if you want that part reverted I'll do so. Keep in mind that that will cause the validation message to display The baz field is required when foo is . instead.

I'd very much like a some second pair of eyes on this. @timacdonald @jessarcher if you're willing?

[jessarcher]

Looks good @driesvints, just thinking this should also be applied to prohibited_if and prohibited_unless for consistency and symmetry?

[driesvints]

@jessarcher Yep indeed. Did that on 8.x. 6.x doesn't have those rules. Thanks Jess!

[JackWH]

@driesvints Does this fix also apply to nullable input array values? I just updated to v8.40.0 and found the second validation rule below failing unexpectedly:

[
    'start_dates'   => 'array|exclude_if:schedule_type,daily|required_if:create_schedule,true',
    'start_dates.*' => 'nullable|exclude_if:schedule_type,daily|required_if:create_schedule,true|date_format:D M jS Y',
]

Validation would report that "The start_dates.123 field is required when create schedule is 1". However, start_dates.* should also be nullable, as it can be excluded based on another input field.

[driesvints]

@JackWH can you please boil down the issue to the most simple way to reproduce it?