TDD: add test

[peterdudfield]

Pull Request

Description

Horrible fix to do with some pandas indexes

Fixes https://github.com/openclimatefix/uk-pv-national-gsp-api/issues/247

How Has This Been Tested?

CI test and ran API locally with this change

• [x] Yes

Checklist:

• [x] My code follows OCF's coding style guidelines
• [x] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [ ] I have checked my code and corrected any misspellings

[codecov[bot]]

Codecov Report

Merging #219 (8674030) into main (4679993) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #219   +/-   ##
=======================================
  Coverage   95.03%   95.04%           
=======================================
  Files          27       27           
  Lines        1693     1694    +1     
=======================================
+ Hits         1609     1610    +1     
  Misses         84       84           

Impacted Files	Coverage Δ
nowcasting_datamodel/__init__.py	100.00% <100.00%> (ø)
nowcasting_datamodel/read/blend/blend.py	96.29% <100.00%> (+0.06%)	:arrow_up:

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