Omit undefineds in serialized objects

[cknitt]

Already discussed this one with @DZakh some time ago, but we did not really come to a conclusion and lost track of it.

When I have an object with a field of type option<'a>, e.g.

@schema
type t = {
  a: string,
  b: option<int>,
}

let obj = {a: "test", b: None}

and I serialize it using S.serializeOrRaiseWith(obj, schema), I get the following JS object:

{a: 'test', b: undefined}

I would prefer to get

{a: 'test'}

i.e., having the field b with value undefined omitted.

I know that b is stripped anyway when serializing the object to JSON using JSON.stringify, but:

1. In other use cases, such as writing the object to IndexedDB, the undefined values are not stripped.

2. The return type of S.serializeOrRaiseWith is JSON.t which is defined as

type rec t =
  | Boolean(bool)
  | @as(null) Null
  | String(string)
  | Number(float)
  | Object(Core__Dict.t<t>)
  | Array(array<t>)

and therefore actually cannot have undefined values.