How do schema types and nullability affect what you test?

Every field and argument in a GraphQL schema has a type, and a trailing ! marks it non-null (required).

type User {
  id: ID!          # always present
  name: String!    # always present
  email: String    # may be null
}
query GetUser($id: ID!) { ... }   # $id is required

What this gives you to test:

• Non-null violations: if a String! field resolves to null, GraphQL must surface an error (and, by spec, nulls can propagate up to the nearest nullable parent). That propagation behaviour is a real, testable edge case.
• Required variables: omitting $id on a $id: ID! operation is a validation error — a cheap, high-value negative test.
• Default values: $limit: Int = 10 should apply the default when the variable is omitted; test both the supplied and omitted paths.

Nullability is where GraphQL's "it's just JSON" black-box testers fall down — the schema is a contract with rules you can assert against.