fix: return error when proof vec is empty instead of submitting empty bytes

[amathxbt]

Problem

ProofSubmitter::submit_proof derives legacy_proof_bytes with:

let legacy_proof_bytes = proofs_bytes.first().cloned().unwrap_or_default();

If proof_result.proofs is empty for any reason, unwrap_or_default() silently substitutes an empty Vec<u8>. That empty bytes slice is then forwarded to the orchestrator as the proof payload. The orchestrator rejects the empty submission, the node loses the task reward, and nothing in the logs indicates that the proof was empty — the submission path returns Ok.

Fix

• Add a NoProofs variant to SubmitError.
• Return Err(SubmitError::NoProofs) immediately when proofs_bytes.is_empty(), before any network call is attempted.

This surfaces the failure clearly in logs and lets analytics correctly attribute it as a local error rather than a submission error.