I've recently spent several hours debugging a problem that was caused by an empty hash definition
```ruby
required do
   hash :hash_name
end
```

Such definition discards any hash content being passed in, which is very
confusing and I reckon rarely useful, but very unintuitive interface
(found other 3 bugs in the codebase caused by the exact same problem).

This change makes such definitions raise, but only if
Mutations.raise_on_empty_hash_filter is set to true (defualt false).
Such optional raising avoids breaking change.

Overall, I recommend changing the interface to raise on bogus definition
as a default, instead of the opt-in.