Update valid times computation to make pv-only possible

[AUdaltsova]

Pull Request

Currently we compute the intersection of non-pv data first. If no PV data is supplied, this throws an error, because it has no data to work with. This change checks if there are any data sources in the non-pv dataset, and skips looking for t0 on other data if not.

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.
Please also list any relevant details for your test configuration

• Yes

If your changes affect data processing, have you plotted any changes? i.e. have you done a quick sanity check?

• 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