fix(unit): always generate change for explanation edit

[nijel]

Proposed changes

It was missed when editing source.

Fixes #12550

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 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.10%. Comparing base (c050f1c) to head (ee9034a).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
weblate/trans/models/unit.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12567   +/-   ##
=======================================
  Coverage   91.10%   91.10%           
=======================================
  Files         591      591           
  Lines       60580    60581    +1     
  Branches     9593     9593           
=======================================
+ Hits        55191    55192    +1     
  Misses       3721     3721           
  Partials     1668     1668           

Files with missing lines	Coverage Δ
weblate/trans/models/unit.py	87.78% <50.00%> (+0.01%)	⬆️