Add support for nullable embedded subobjects

[milessabin]

Currently embedded subobjects are assumed to be non-null. There are workarounds for situations which require nullability, but these aren't ideal and it would be better to provide first-class support.

[tpolecat]

Our hack https://github.com/gemini-hlsw/lucuma-odb/blob/40e09a581ce3addbed94ebf33ac7e197d0d6d5b5/modules/service/src/main/scala/lucuma/odb/graphql/table/CodecExtensions.scala