dont error on first batch - this is when the batches are shuffled

[peterdudfield]

Pull Request

Description

• small fix to batch
• include some legacy code for v15

How Has This Been Tested?

• normal unittests

• ran ML using this new code

• [ ] Yes

Checklist:

• [ ] My code follows OCF's coding style guidelines
• [ ] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] 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-commenter]

Codecov Report

Merging #639 (43bd4f8) into main (15c56c2) will decrease coverage by 0.17%. The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main     #639      +/-   ##
==========================================
- Coverage   93.32%   93.14%   -0.18%     
==========================================
  Files          45       45              
  Lines        3041     3049       +8     
==========================================
+ Hits         2838     2840       +2     
- Misses        203      209       +6     

Impacted Files	Coverage Δ
nowcasting_dataset/dataset/batch.py	94.24% <33.33%> (-4.23%)	:arrow_down:

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