xrpc: "message" field on errors should be optional, not nullable #38

i think codegen is missing a [serde(skip_serializing_if = "Option::is_none")] for xrpc error types' message field.

the spec says

• message (string, optional): description of the error, appropriate for display to humans

so i think rendering a null is non-compliant.

other codegen stuff in the ecosystem (like bsky's typescript lex) seems to error out on them