[11.x] Add NullableIf Validation Rule

[mohammadsalehnia]

This pull request introduces a new validation rule, NullableIf, to the Laravel framework. The NullableIf rule allows an attribute to be nullable based on a condition. If the condition evaluates to true, the attribute can be null.

Details:

File Modified: src/Illuminate/Validation/Rules/NullableIf.php
Functionality:
Constructor: Accepts a callable or boolean condition.
__toString Method: Converts the rule to a validation string based on the condition.
Usage Example:

In a request validation rule:
'field' => [ Rule::nullableIf(true), 'string' ],

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!