Allow null in GraphQL response

[wilsonianb]

Replace maybeValue with inputMaybeValue (available in newer versions of graphql-code-generator) to allow null in GraphQL responses. https://github.com/interledger/rafiki/blob/main/packages/backend/codegen.yml#L11

[wilsonianb]

This is also necessary to return null from a __resolveType resolver https://www.apollographql.com/docs/apollo-server/v2/schema/unions-interfaces/#resolving-a-union

[wilsonianb]

:confused: We appear to already be able to return null from resolvers. I even tried rebasing on:

• https://github.com/interledger/rafiki/pull/921

[wilsonianb]

This is also necessary to return null from a __resolveType resolver https://www.apollographql.com/docs/apollo-server/v2/schema/unions-interfaces/#resolving-a-union

And we don't current have any unions in our schema