Fix pick_winner crash on empty lottery pool

[sueun-dev]

Closes #8

This change prevents pick_winner from crashing when candidate blocks exist but no validator has positive stake. It now skips that round safely.

Step	Before fix	After fix
Reproduction command	env PYTHONPATH=src .venv/bin/python repro_pick_winner.py (inline script used below)	same command
Key output	task_done= True, task_exception= IndexError('list index out of range')	task_done= False, task_exception= None
Test result	Manual reproduction shows crash (fail)	sandbox-exec -f /tmp/no-network.sb env PYTHONPATH=src .venv/bin/pytest -q -> 2 passed (pass)

Reproduction script used:

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()) if t.done() else None)

asyncio.run(main())