Issue/no nan adjust mw

[peterdudfield]

Pull Request

Description

If there are no ME values, then default to 0. This will allow longer National_xg model to be deployed

How Has This Been Tested?

• added a test, and checked behaviour

• 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]

Codecov Report

Merging #195 (f9e2a71) into main (e01682f) will decrease coverage by 0.03%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##             main     #195      +/-   ##
==========================================
- Coverage   94.82%   94.79%   -0.03%     
==========================================
  Files          27       27              
  Lines        1603     1613      +10     
==========================================
+ Hits         1520     1529       +9     
- Misses         83       84       +1     

Impacted Files	Coverage Δ
nowcasting_datamodel/save/adjust.py	93.33% <81.81%> (-1.01%)	⬇️
nowcasting_datamodel/__init__.py	100.00% <100.00%> (ø)
nowcasting_datamodel/fake.py	100.00% <100.00%> (ø)
nowcasting_datamodel/read/read_metric.py	100.00% <100.00%> (ø)

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