[8.x] Add default rules in conditional rules

[bastien-phi]

PR #38361 introduced ConditionalRules to conditionally add rules to the validation.

Imagine a validation like :

public function rules()
{
    return [
        'email' => [
            Rule::when($this->someComplexTest(), ['required']),
            Rule::when(!$this->someComplexTest(), ['nullable']),
        ]
    ];
}

This PR adds the ability to define the behaviour when the condition is false.

public function rules()
{
    return [
        'email' => [
            Rule::when($this->someComplexTest(), ['required'], ['nullable']),
        ]
    ];
}

This behavior already exists in collections, conditionable, ...

[bastien-phi]

Thanks !