Make the reason argument in `@deprecated` non-nullable

At a glance

• Identifier: #1040
• Stage: RFC 3 / Accepted
• Champion: @martinbonnin
• Latest activity: Spec PR merged on 2024-12-05
• Spec PR: https://github.com/graphql/graphql-spec/pull/1040
• Related:
  • #53

Spec PR description

Follow up from https://github.com/graphql/graphql-spec/issues/53#issuecomment-1688335159

Make reason non-nullable:

directive @deprecated(
  reason: String! = "No longer supported"
) on FIELD_DEFINITION | ARGUMENT_DEFINITION | INPUT_FIELD_DEFINITION | ENUM_VALUE

This is technically a breaking change for someone that does this:

type Foo {
  bar: String! @deprecated(reason: null)
}

But feels like this shouldn't be allowed in the first place?

Fixes https://github.com/graphql/graphql-spec/issues/53