logging: don't crash if backend is uninitialized

[half-duplex]

Description

When core.logging_channel is used, errors early in startup cause an exception that halts the bot. (bdc10e0, current master)

Repro

sopel.cfg

[core]
host = fake.local
logging_channel = #services
owner = mal
enable = version
foo = bar

Start the bot: sopel start -c sopel.cfg

Current Result

  File "sopel/logger.py", line 32, in emit
    if not self._bot.backend.is_connected():
AttributeError: 'NoneType' object has no attribute 'is_connected'

full logs

(env) [mal@nova sopel-test]$ sopel start -c sopel.cfg
Sopel 7.1.0.dev0 (running on Python 3.8.5)
https://sopel.chat/

Loaded config file: /home/mal/sources/sopel-test/sopel.cfg
[2021-01-23 03:18:54,137] sopel.bot            INFO     - Loading plugins...
[2021-01-23 03:18:54,142] sopel.bot            INFO     - Plugin loaded: version
[2021-01-23 03:18:54,149] sopel.bot            INFO     - Plugin loaded: coretasks
[2021-01-23 03:18:54,150] sopel.bot            INFO     - Registered 1 plugins, 0 failed, 39 disabled
[2021-01-23 03:18:54,150] sopel.bot            WARNING  - Config option `core.foo` is not defined by its section and may not be recognized by Sopel.
Unexpected error in bot setup
Traceback (most recent call last):
  File "/home/mal/sources/sopel-test/env/bin/sopel", line 8, in <module>
    sys.exit(main())
  File "/home/mal/sources/sopel-test/env/lib/python3.8/site-packages/sopel/cli/run.py", line 714, in main
    return command(opts)
  File "/home/mal/sources/sopel-test/env/lib/python3.8/site-packages/sopel/cli/run.py", line 439, in command_start
    ret = run(settings, pid_file_path)
  File "/home/mal/sources/sopel-test/env/lib/python3.8/site-packages/sopel/cli/run.py", line 75, in run
    p.setup()
  File "/home/mal/sources/sopel-test/env/lib/python3.8/site-packages/sopel/bot.py", line 280, in setup
    self.post_setup()
  File "/home/mal/sources/sopel-test/env/lib/python3.8/site-packages/sopel/bot.py", line 369, in post_setup
    LOGGER.warning(
  File "/usr/lib/python3.8/logging/__init__.py", line 1446, in warning
    self._log(WARNING, msg, args, **kwargs)
  File "/usr/lib/python3.8/logging/__init__.py", line 1577, in _log
    self.handle(record)
  File "/usr/lib/python3.8/logging/__init__.py", line 1587, in handle
    self.callHandlers(record)
  File "/usr/lib/python3.8/logging/__init__.py", line 1649, in callHandlers
    hdlr.handle(record)
  File "/usr/lib/python3.8/logging/__init__.py", line 950, in handle
    self.emit(record)
  File "/home/mal/sources/sopel-test/env/lib/python3.8/site-packages/sopel/logger.py", line 32, in emit
    if not self._bot.backend.is_connected():
AttributeError: 'NoneType' object has no attribute 'is_connected'
(env) [mal@nova sopel-test]$ 

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches