fix: handle first nonce in orchestrator

[rach-id]

Overview

Just to avoid having an ugly log when starting the orchestrator since by definition, there is no prior valset to nonce 1:

core0-orch    | I[2023-10-12|00:07:52.579] processing nonce                             nonce=1
core0-orch    | D[2023-10-12|00:07:52.580] failed to query last valset before nonce (most likely pruned). signing anyway err="rpc error: code = Unknown desc = codespace qgb code 28: there is no valset before attestation nonce 1"

Checklist

• New and updated code has appropriate documentation
• New and updated code has new and/or updated testing
• Required CI checks are passing
• Visual proof for any user facing features like CLI or documentation updates
• Linked issues closed with keywords