Do not render emplty field list

[Fluxx]

According to the GraphQL object spec, for a given type Foo the fields definition for that type is optional. As far as I can tell, if you want to not specify a fields list, you need to also omit the wrapping {...}. i.e:

// This is invalid
type Foo {
}

// This is valid
type Foo

Currently, Caliban always prints schema with the wrapping {...}, even if there are no fields for the type, which I believe is invalid. I tested this against our schema in Apollo and saw the below. Sensitive parts omitted, but the important part is the AST parse failure.

$ echo 'schema {query: Q} type Q {}' | rover subgraph check [...]
Checking the proposed schema for subgraph [...]
error: Could not parse partial schema as AST.

$ echo 'schema {query: Q} type Q' | rover subgraph check [...]
Checking the proposed schema for [...]
Compared 108 schema changes against 25 operations

[ghostdogpr]

Looks okay! The CI failed because of formatting, can you run fmt in sbt?

[Fluxx]

Will do.

Also, we ran into a similar problem where an empty schema also causes issues, where both schema and schema {} are invalid. I will see if I can make that change as part of this PR as well.

[Fluxx]

Ran fmt and also added some behavior to handle empty schemas. Please take a look!

[ghostdogpr]

Thanks!