Fix eval parameters omit null description

[Pedro Mora (stretpjc)]

Summary

Remote eval dev mode (braintrust eval <file> --dev) hid all evals in the playground UI when any eval declared a parameter without a description.

All three branches of serialize_eval_parameters (prompt, model, data) emitted "description": null when unset. The playground's zod schema only accepts a string or an absent key, and it validates the whole /list response in one
safeParse — so one description-less parameter hid every eval on the endpoint.

Fix: include description only when it's not None, same as default already is. #508 fixed this pattern for prompt defaults; description was missed.

Test

With proposed change the remote eval is detected:

Same remote eval running in 0.30.1 is not detected:

[Pedro Mora (stretpjc)]

Forgot to mention the linear:
https://linear.app/braintrustdata/issue/SDK-92/remote-eval-params-with-no-description-serialize-description-null