Can required lexicon fields have null or false-y values?

[snarfed]

Minor nitpicky question: https://atproto.com/specs/lexicon#object says:

required (array of strings, optional): indicates which properties are required

So the field must be present. Can its value be null? Or false-y, eg the empty string?

(This came up recently in the API Discord 😁)

[snarfed]

@DavidBuchanan314 says this probably falls through to nullable. Makes sense if so!

[bnewbold]

If a field is required, the key must be present in the JSON/CBOR object.

If a field is nullable, it may have have null as the value.

We don't treat "false-y" as a thing anywhere, I think: null is distinct from field not existing, which is distinct from things like empty-string, 0, or empty array.

Note that there is also a null field type.

Per the specs today, it is allowed to be both not-required and nullable, but we really strongly recommend against this. There are one or two instances of this in the current Lexicons, and we would like to remove them and update the spec to say that a field can either be optional or not-required but not both. The main motivation for this is the Go JSON marshalling library in the Go standard library, which does not have a way of representing "double-optional": if the Go value is null it either needs to serialize as "field doesn't exist at all", or null, there isn't a way to select the behavior at runtime. This is mentioned in #2128 as "Nullable and Optional".