Handle empty file error

[fan-tom]

Closes #2800

• Tests added for any new code
• Ran make fmt-fix (or had formatting run automatically on all files edited)
• Documentation added for any new functionality
• Entries added to ./unreleased/ for any new functionality

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (003302f) 78.89% compared to head (f5da1a9) 78.91%.

Files	Patch %	Lines
...ala/at/forsyte/apalache/tla/imp/SanyImporter.scala	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2821      +/-   ##
==========================================
+ Coverage   78.89%   78.91%   +0.02%     
==========================================
  Files         467      467              
  Lines       15942    15944       +2     
  Branches     2589     2585       -4     
==========================================
+ Hits        12577    12583       +6     
+ Misses       3365     3361       -4     

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[shonfeder]

Excellent. Thank you!

I'll add the changelog entry and merge this in 👯