add Optional in metric values

[peterdudfield]

Pull Request

Description

Allow Nonw for time_of_day

Fixes https://github.com/openclimatefix/uk-analysis-dashboard/issues/110

How Has This Been Tested?

Ran analysis dashbaord locally

• [ ] 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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.31%. Comparing base (4ae9dc2) to head (d13163b). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #264 +/- ## ======================================= Coverage 95.31% 95.31% ======================================= Files 27 27 Lines 1770 1770 ======================================= Hits 1687 1687 Misses 83 83 ```

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