fix: remove zero-fill in singlediode DC model to preserve NaNs (Close…

[Mohamedhassan268]

This PR addresses Issue #1409. I removed the .fillna(0) behavior inside ModelChain._singlediode

[github-actions]

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[cwhanse]

@Mohamedhassan268 I doubt there is an existing test in test_modelchain.py that includes nan in the weather input. I am concerned that removing the fillna(0) will uncover other problems.

Can you make a new test for test_modelchain.py that checks for proper handling of nan in weather input? Just so we can see what may go wrong?

In the end I think we'll want to add nan to the weather fixture. However, that's not a simple task because test_modelchain.py has a lot of tests that use the weather fixture, and I suspect that some tests rely on the index length of 2 to match weather with other data.

[cwhanse]

Please undo this delete - the comment is about the following if statement.