return array of nans when there is no pv data

[peterdudfield]

Pull Request

Description

Return array of nans instead of no data, when there are no pv yields The pv systems is filled with 0, if elevation is below -10. This helps for live data, so that not all the data is removed, when removing nans

Fixes https://github.com/openclimatefix/nowcasting_forecast/issues/122

How Has This Been Tested?

normal unitest tests, + extra ones

• [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
• [x] I have checked my code and corrected any misspellings