Fix handling of `NoneType` for torrents in `count_torrents_in_states` function

[DAcodedBEAT]

Proposed change

Added a check to handle cases where the 'torrents' data is None, avoiding a TypeError when attempting to get the length of a NoneType object. The function now returns 0 if 'torrents' is None, ensuring robust behavior when no torrent data is available.

Here was the traceback when looking at the home assistant logs:

Traceback (most recent call last):
  File "/usr/local/lib/python3.12/site-packages/homeassistant/helpers/entity_platform.py", line 595, in _async_add_entities
    await coro
  File "/usr/local/lib/python3.12/site-packages/homeassistant/helpers/entity_platform.py", line 909, in _async_add_entity
    await entity.add_to_platform_finish()
  File "/usr/local/lib/python3.12/site-packages/homeassistant/helpers/entity.py", line 1366, in add_to_platform_finish
    self.async_write_ha_state()
  File "/usr/local/lib/python3.12/site-packages/homeassistant/helpers/entity.py", line 1005, in async_write_ha_state
    self._async_write_ha_state()
  File "/usr/local/lib/python3.12/site-packages/homeassistant/helpers/entity.py", line 1130, in _async_write_ha_state
    self.__async_calculate_state()
  File "/usr/local/lib/python3.12/site-packages/homeassistant/helpers/entity.py", line 1067, in __async_calculate_state
    state = self._stringify_state(available)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/homeassistant/helpers/entity.py", line 1011, in _stringify_state
    if (state := self.state) is None:
                 ^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/homeassistant/components/sensor/__init__.py", line 542, in state
    value = self.native_value
            ^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/homeassistant/components/qbittorrent/sensor.py", line 170, in native_value
    return self.entity_description.value_fn(self.coordinator)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/homeassistant/components/qbittorrent/sensor.py", line 101, in <lambda>
    value_fn=lambda coordinator: count_torrents_in_states(coordinator, []),
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/homeassistant/components/qbittorrent/sensor.py", line 181, in count_torrents_in_states
    return len(torrents)
           ^^^^^^^^^^^^^
TypeError: object of type 'NoneType' has no len()

Type of change

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

Additional information

• This PR fixes or closes issue: fixes #121358
• This PR is related to issue: #121358
• Link to documentation pull request:

Checklist

• [ ] The code change is tested and works locally.
• [ ] Local tests pass. Your PR cannot be merged unless tests pass
• [x] There is no commented out code in this PR.
• [ ] I have followed the development checklist
• [x] I have followed the perfect PR recommendations
• [ ] The code has been formatted using Ruff (ruff format homeassistant tests)
• [ ] Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• [x] Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• [x] The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• [x] New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• [x] For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.

To help with the load of incoming pull requests:

• [x] I have reviewed two other open pull requests in this repository.

[home-assistant[bot]]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks :+1:

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[home-assistant[bot]]

Hey there @geoffreylagaisse, @finder39, mind taking a look at this pull request as it has been labeled with an integration (qbittorrent) you are listed as a code owner for? Thanks!

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[finder39]

Clean and simple. Nicely done and thank you!

[DAcodedBEAT]

Clean and simple. Nicely done and thank you!

@finder39 thanks for the quick review! how long do these PRs typically take to get approved and merged by core maintainers?

I would love to see less stacktraces in my logs 😅

[joostlek]

I would love to see less stacktraces in my logs

Workaround is to start downloading more torrents right :P

[finder39]

Clean and simple. Nicely done and thank you!

@finder39 thanks for the quick review! how long do these PRs typically take to get approved and merged by core maintainers?

I would love to see less stacktraces in my logs 😅

It's not too long as you can see, especially on smaller PRs like this :)