FilterToSchema doesn't remove empty selection sets

[adamkl]

Intended outcome:
Given target schema:

type Query {
  customer: Customer
}

type Customer {
  name: String
  address: Address
}

type Address {
  street: String
  city: String
  country: String
}

And query:

query CustomerQuery {
  customer {
    name
    address {
      planet
    }
  }
}

FilterToSchema leaves behind an empty selection set which fails validation on a remote graphql server:

query CustomerQuery {
  customer {
    name
    address
  }
}

Actual outcome:
FilterToSchema should remove fields with empty selection sets:

query CustomerQuery {
  customer {
    name
  }
}

How to reproduce the issue:
I'm preparing a PR based on this branch which includes unit tests that reproduce the error and some code changes to resolve it.

[adamkl]

As an addendum to this, I'm sure there are other scenarios that I haven't accounted for (empty selection sets on inline fragments?), and I can see the code for this getting more complex over time. A friend of mine (@brysgo) tackled this problem by leveraging GraphQL's built in validation. It might be prudent to take a look at his approach (graphql-mask)