fix: gracefully handle uninstalled checks

[nijel]

Proposed changes

When check is no longer installed, Weblate can not get its object and processs it.

Fixes #13020

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information

[codecov]

Codecov Report

Attention: Patch coverage is 58.33333% with 5 lines in your changes missing coverage. Please review.

Project coverage is 90.92%. Comparing base (5607205) to head (3b69c89).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
weblate/checks/views.py	0.00%	2 Missing and 1 partial ⚠️
weblate/trans/views/js.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13184      +/-   ##
==========================================
- Coverage   90.93%   90.92%   -0.01%     
==========================================
  Files         601      601              
  Lines       62148    62156       +8     
  Branches     6404     6406       +2     
==========================================
+ Hits        56512    56515       +3     
- Misses       3975     3978       +3     
- Partials     1661     1663       +2     

Files with missing lines	Coverage Δ
weblate/checks/models.py	82.55% <100.00%> (ø)
weblate/trans/tests/test_edit.py	100.00% <100.00%> (ø)
weblate/trans/views/js.py	85.96% <33.33%> (-3.13%)	⬇️
weblate/checks/views.py	64.82% <0.00%> (-1.37%)	⬇️