[DBAL-577] Fix platform's IN expression

`AbstractPlatform::getInExpression()` calls an undefined method `AbstractPlatform::getIdentifiers()`. This PR removes this call and also removes the raising of an exception in case of empty given values. The logic does not work when passing a string as `$values` parameter and furthermore other expression methods do not check the input parameters either. Besides that MySQL for example also accepts empty `IN()` expressions.