Fix a bug where a socket connection to "bsv.aftrek.org" is successful…

[AustEcon]

…ly established but on protocol negotiation, the result is "None"

• This crashes the entire asyncio event loop with "TypeError: cannot unpack non-iterable NoneType object"

Context:

This is the relevant traceback:

2023-08-10 12:43:42,021:ERROR:async:async task raised an unhandled exception
Traceback (most recent call last):
  File ".../electrumsv/electrumsv/async_.py", line 94, in _collect
    future.result()
  File ".../.pyenv/versions/3.9.16/lib/python3.9/concurrent/futures/_base.py", line 439, in result
    return self.__get_result()
  File ".../.pyenv/versions/3.9.16/lib/python3.9/concurrent/futures/_base.py", line 391, in __get_result
    raise self._exception
  File ".../electrumsv/electrumsv/network.py", line 944, in _main_task
    await group.spawn(self._monitor_wallets, group)
  File ".../electrumsv/lib/python3.9/site-packages/aiorpcx/curio.py", line 255, in aexit
    await self.join()
  File ".../electrumsv/lib/python3.9/site-packages/aiorpcx/curio.py", line 213, in join
    raise task.exception()
  File ".../electrumsv/electrumsv/network.py", line 973, in _maintain_connections
    await group.spawn(self._maintain_connection, n)
  File ".../electrumsv/lib/python3.9/site-packages/aiorpcx/curio.py", line 255, in aexit
    await self.join()
  File ".../electrumsv/lib/python3.9/site-packages/aiorpcx/curio.py", line 213, in join
    raise task.exception()
  File ".../electrumsv/electrumsv/network.py", line 987, in _maintain_connection
    await server.connect(self, n)
  File "...electrumsv/electrumsv/network.py", line 266, in connect
    await session.run()
  File ".../electrumsv/electrumsv/network.py", line 763, in run
    await self._negotiate_protocol()
  File ".../electrumsv/electrumsv/network.py", line 463, in _negotiate_protocol
    server_string, protocol_string = await self.send_request(method, args)
TypeError: cannot unpack non-iterable NoneType object

This only occurs for the "bsv.aftrek.org" server and is strange because we've never encountered this behaviour from any ElectrumX server in the past.

To reproduce this in isolation sheds some light:

import asyncio
import aiorpcx
import ssl

async def main(host, port):
    async with aiorpcx.connect_rs(host, port, ssl=ssl.SSLContext(ssl.PROTOCOL_TLS)) as session:
        print("Connected!")
        result = await session.send_request('server.version', ("1.3.16", ["1.4", "1.4.2"]))
        print(result)

asyncio.run(main('sv3.satoshi.io', 50002))
asyncio.run(main('bsv.aftrek.org', 50002))

Console output:

Connected!
['ElectrumX 1.20.2', '1.4.2']
Connected!
None

So a connection is still being established to bsv.aftrek.org but for some reason protocol negotiation is failing.