Handle missing section ID for Plex clips

[jjlawren]

Proposed change

I've noticed that clips sometimes do not have a library section ID, likely because they're remote/external sources.

Example traceback:

2021-06-07 17:20:31 ERROR (MainThread) [homeassistant] Error doing job: Task exception was never retrieved
Traceback (most recent call last):
  File "/src/home-assistant/homeassistant/components/plex/server.py", line 280, in async_update_session
    await self.async_update_platforms()
  File "/src/home-assistant/homeassistant/helpers/debounce.py", line 78, in async_call
    await task
  File "/src/home-assistant/homeassistant/components/plex/server.py", line 496, in _async_update_platforms
    await self.hass.async_add_executor_job(sync_tasks)
  File "/.pyenv/versions/3.8.6/lib/python3.8/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/src/home-assistant/homeassistant/components/plex/server.py", line 494, in sync_tasks
    process_sessions()
  File "/src/home-assistant/homeassistant/components/plex/server.py", line 474, in process_sessions
    self.active_sessions[unique_id] = PlexSession(self, session)
  File "/src/home-assistant/homeassistant/components/plex/models.py", line 62, in __init__
    self.update_media(session)
  File "/src/home-assistant/homeassistant/components/plex/models.py", line 81, in update_media
    elif media.librarySectionID < 1:
TypeError: '<' not supported between instances of 'NoneType' and 'int'

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

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• This PR fixes or closes issue: fixes #
• This PR is related to issue:
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