Hasura bug: single-result computed fields return arrays

[bitjson]

Currently transactions.authchains.migrations.transaction returns an array even though it should never have more than one result. Described in detail here: https://github.com/hasura/graphql-engine/issues/4299#issuecomment-817040370

Open issue: https://github.com/hasura/graphql-engine/issues/6562

(Related and also an issue for some of our planned APIs: https://github.com/hasura/graphql-engine/issues/4752)