Adds guard for persisters returning bad state

[skrawcz]

The entire call from load_state should be None if something isn't found.

Otherwise the persister should fix its assumptions/make sure None state is not saved.

Changes

• ApplicationBuilder code
• test for it

How I tested this

• locally

Notes

• Spurred by discussion in #146

Checklist

• [ ] PR has an informative and human-readable title (this will be pulled into the release notes)
• [ ] Changes are limited to a single goal (no scope creep)
• [ ] Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• [ ] Any change in functionality is tested
• [ ] New functions are documented (with a description, list of inputs, and expected output)
• [ ] Placeholder code is flagged / future TODOs are captured in comments
• [ ] Project documentation has been updated if adding/changing functionality.