Minor amendment to obs_vars handling

[jmorris-uk]

Description

Fix bug in obs-var checking in input files.

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 47.84%. Comparing base (c9a2074) to head (b367b39).

Files with missing lines	Patch %	Lines
process/main.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4137      +/-   ##
==========================================
- Coverage   47.84%   47.84%   -0.01%     
==========================================
  Files         141      141              
  Lines       29754    29755       +1     
==========================================
  Hits        14236    14236              
- Misses      15518    15519       +1     

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