fix pvwatts_ac exception and return value when called with pdc=0s

[wholmgren]

• Closes pvwatts_ac raises ZeroDivisionError for scalar pdc=0, RuntimeWarning for array(0) input #675
• I am familiar with the contributing guidelines.
• Fully tested. Added and/or modified tests to ensure correct behavior for all reasonable inputs. Tests (usually) must pass on the TravisCI and Appveyor testing services.
• Updates entries to docs/sphinx/source/api.rst for API changes.
• Adds description and name entries in the appropriate docs/sphinx/source/whatsnew file for all changes.
• Code quality and style is sufficient. Passes LGTM and SticklerCI checks.
• New code is fully documented. Includes sphinx/numpydoc compliant docstrings and comments in the code where necessary.
• Pull request is nearly complete and ready for detailed review.

[mikofski]

very clever usage of np.divide(..., where=...)

[wholmgren]

@mikofski a stack overflow user gets the credit, as usual!

[wholmgren]

@cwhanse go ahead and merge if this looks good to you too.