Alternative fix to #1460

Since array_values() is used when grabbing arguments in
[AreAllArgumentsValidValidator::processArgumentValidation()](https://github.com/phpDocumentor/phpDocumentor2/blob/develop/src/phpDocumentor/Plugin/Core/Descriptor/Validator/Constraints/Functions/AreAllArgumentsValidValidator.php#L83), the "index" value of 0
would be seen as empty in the [__set()](https://github.com/phpDocumentor/phpDocumentor2/blob/develop/src/phpDocumentor/Plugin/Core/Descriptor/Validator/ValidationValueObject.php#L43) function.

Initializing array keys "index" (and "key" unless this isn't needed) to 0 should fix this issue, thus removing the need to cast $value->index to an integer.