Cover empty field-selections (#4228)
Closes https://github.com/graphql/graphql-js/issues/3790
This follows the spec closer to the letter as we consider empty
selection-sets invalid as well. This however does not cover empty
operation-definitions, which should also be considered invalid.
The parser itself considers empty-selections invalid as it requires you
to specify a name before a closing curly, hence why testing happens a
bit more manual here. Despite our parser not considering this valid, I
think this is good to validate when folks use a separate parser or
construct documents manually as this is valid in our type-system.
