freeze: Pydantic emits `description: null` instead of omitting it

[williballenthin]

i think we should avoid emitting description: null when a node doesn't have one, rather than including the empty value in the Freeze output. it'll make the file a tiny bit smaller and easier to inspect.

this is potentially a breaking change, but extremely minor, i think.

[abhinav-phi]

Hi @williballenthin, I'd like to work on this issue.

I'll investigate the current Pydantic serialization, update it to omit the description field when it is None instead of emitting description: null, and add or update tests to ensure the Freeze output remains correct.

[williballenthin]

no