allowing :empty on integer filter

I'm having a problem with specific scenario (which is very likely to rails env). If I want rails to set default value (set on database level) to some field, we need to pass empty string to it, like this: `User.create(name: "")`

While this works for strings, it does't work with integers since mutations will raise an error if empty string is passed to integer. This simple patch will fix that. Don't know whether it could have different implications...