rust-sdk: a failed task records a null traceback, not an empty one

[kartikeya-27]

crates/flexiq/src/outcome.rs:42 writes:

serde_json::json!({
    "errtype": "TaskError",
    "message": err.message,
    "traceback": serde_json::Value::Null,
})

BINDING_CONTRACT.md says the key is required and its type is an array of
strings, [] when the language or runtime cannot provide frames. null is a
third shape, and the Rust SDK is the only shell that writes it.

It is tolerated by most readers and not by all — see the companion issue against
the Go client, which reads it as unstructured and loses the errtype with it. A
reader that has to special-case a shape the contract does not describe is a
reader that will eventually not.

Fix is one literal, Value::Null to an empty array, plus the formatter test.

Worth checking whether the doc comment above it should change too: it explains
that errtype is the constant "TaskError" and traceback is null because
Rust has no exception type to name and no stack trace to attach. The first half
stays true; the second is an argument for [], which is exactly what the
contract says the no-frames case looks like.