I'd like a nicer api to create objects without null properties

I often write the following
```rs
json.object([
#("foo", json.nullable(x, json.int)
])
```
Which will create `{foo: null}` when x is `None`. What I often want is `{}` when x is null several API's in the wild crash if an optional field is instead sent with a value of null.

A possible implementation was suggested in https://github.com/gleam-lang/json/pull/31 though it's not clear this is the best PR.

Start by reviewing the object-building example in the issue and the proposed implementation in PR 31. Determine the API shape that best supports omitting optional fields whose values are None; done means non-null fields remain and null optional fields are omitted from the resulting object.

Written by the indexing model from the issue text.