pick_winner crashes when no validator has stake

[sueun-dev]

When a candidate block is present but no validator has a positive stake entry, the lottery pool is empty and pick_winner crashes.

Reproduction:

env PYTHONPATH=src .venv/bin/python - <<'PY'
import asyncio
from p2p import peer

async def main():
peer.validators.clear()
peer.tempblocks.clear()
peer.tempblocks.append({"Validator": "missing", "Index": 1, "BPM": 60, "Timestamp": "t", "PrevHash": "h", "Hash": "h2"})
q = asyncio.Queue()
t = asyncio.create_task(peer.pick_winner(q))
await asyncio.sleep(1.2)
print('task_done=', t.done())
print('task_exception=', repr(t.exception()))

asyncio.run(main())
PY

Observed output:

• task_done= True
• task_exception= IndexError('list index out of range')

Expected behavior:

• The round should be skipped safely when no eligible validator exists.