fix: Handle null reference when Voice State is still in cache, but the channel it belongs to has been expired

[silasary]

Pull Request Type

• Feature addition
• Bugfix
• Documentation update
• Code refactor
• Tests improvement
• CI/CD pipeline enhancement
• Other: [Replace with a description]

Description

If you have a particularly small expiry for the channel cache, or someone stays in a Voice Channel for a particularly long time, you can end up with a situation where the Voice State is cached, but the associated channel is not.
This leads to a fun null reference exception when the user eventually does leave the channel.

Changes

Adds a check that state.channel is not None.

Related Issues

Fixes #1644

Test Scenarios

Python Compatibility

• I've ensured my code works on Python 3.10.x
• I've ensured my code works on Python 3.11.x

Checklist

• I've run the pre-commit code linter over all edited files
• I've tested my changes on supported Python versions
• I've added tests for my code, if applicable
• I've updated / added documentation, where applicable

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 46.11%. Comparing base (af9866f) to head (3bcaaee).
Report is 2 commits behind head on unstable.

Files	Patch %	Lines
interactions/client/smart_cache.py	0.00%	1 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff            @@
##           unstable    #1648   +/-   ##
=========================================
  Coverage     46.11%   46.11%           
=========================================
  Files           147      147           
  Lines         15638    15638           
=========================================
  Hits           7211     7211           
  Misses         8427     8427           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.