Prevent empty fields from rendering as "None"

[codesue]

What does this pull request do?

This change fixes a bug that made empty objects render as "None" by:

• defining a __len__() for model card fields that ignores None values recursively when counting items, meaning empty fields and nested empty fields will be falsy
• adding if statements to default templates as needed

This also fixes a bug where nested empty were being serialized when writing a model card to a proto since they were truthy.

Example usage of len() and bool():

>>> from model_card_toolkit.model_card import ModelCard
>>> model_card = ModelCard()
>>> len(model_card)
0
>>> bool(model_card)
False
>>> model_card.model_details.name = "Placeholder"
>>> len(model_card)
1
>>> bool(model_card)
True

If you're recreating a model card from a proto, use if model_card is None instead of if not model_card to test whether the operation was successful if you want to fail on None but not on ModelCard().

Fixes #213

How did you test this change?

• Ran all unit tests
• Ran the MLMD demo notebook and rendered a n HTML model card
• Ran the scikit-learn demo notebook and rendered a Markdowm model card

Screenshot from the offical MLMD Model Card Toolkit Demo:

Screenshot from running the demo notebook with this fix implemented:

How did you document this change?

Docstrings

Before submitting

Before submitting a pull request, please be sure to do the following:

• Read the How to Contribute guide if this is your first contribution.
• Open an issue or discussion topic to discuss this change.
• Write new tests if applicable.
• Update documentation if applicable.

[codesue]

Hi @casassg, @deutranium, or @hanneshapke, would you please review this fix when you get a chance. 🙏