Empty dataframe instead of creating with an undefined variable

[andrewfullard]

📝 Description

Type: 🪲 bugfix

Fixes #274

🚦 Testing

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☑️ Checklist

• I requested two reviewers for this pull request
• I updated the documentation according to my changes
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[codecov]

Codecov Report

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

Project coverage is 68.59%. Comparing base (fc47fbd) to head (472d4d9).
Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
carsus/io/cmfgen/base.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #402      +/-   ##
==========================================
- Coverage   68.87%   68.59%   -0.28%     
==========================================
  Files          31       31              
  Lines        3733     3828      +95     
==========================================
+ Hits         2571     2626      +55     
- Misses       1162     1202      +40     

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[github-actions]

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