Handle empty node list in `PreferredPeerPool`

[pipermerriam]

What is wrong?

This happens

   ERROR  05-23 16:09:45     service  Unexpected error in <p2p.peer.PreferredNodePeerPool object at 0x10f68bc88>, exiting
Traceback (most recent call last):
  File "/Users/piper/sites/py-evm/p2p/service.py", line 36, in run
    await self._run()
  File "/Users/piper/sites/py-evm/p2p/peer.py", line 697, in _run
    await self.maybe_connect_to_more_peers()
  File "/Users/piper/sites/py-evm/p2p/peer.py", line 763, in maybe_connect_to_more_peers
    await self._connect_to_nodes(self.get_nodes_to_connect())
  File "/Users/piper/sites/py-evm/p2p/peer.py", line 777, in _connect_to_nodes
    for node in nodes:
  File "/Users/piper/sites/py-evm/p2p/peer.py", line 954, in get_nodes_to_connect
    yield from self.discovery.get_random_nodes(num_nodes_needed)
  File "/Users/piper/sites/py-evm/p2p/kademlia.py", line 240, in get_random_nodes
    node = random.choice(bucket.nodes)
  File "/Users/piper/python-environments/trinity/lib/python3.6/random.py", line 257, in choice
    raise IndexError('Cannot choose from an empty sequence') from None
IndexError: Cannot choose from an empty sequence

How can it be fixed

Gracefully handle this situation.