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 Handling case when state is None #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.

[elijahbenizzy]

Looks good