fix: struct field non-nullable type issue.

[Genesis929]

This issue is related to pypi.ipynb.

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes b/361145903 🦕

[shobsi]

Let's find a way to write a system test to showcase the value of this change yaqs/4269233226351902720 in a later PR