[MAINTENANCE] Optionally allow None values in RenderedAtomicContent

[NathanFarmer]

Changes proposed in this pull request:

• Optionally allow None values in RenderedAtomicContent
• Call str on RendererValueType during serialization

Definition of Done

• My code follows the Great Expectations style guide
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added unit tests where applicable and made sure that new and existing tests are passing.
• I have run any local integration tests and made sure that nothing is broken.

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[dctalbot]

Thanks! Capturing the string output seems like be a good candidate for snapshot testing if you have that in place.

[NathanFarmer]

Thanks! Capturing the string output seems like be a good candidate for snapshot testing if you have that in place.

What string output are you referring to? The rendered content can change, it is not static. Snapshots tests are probably the reason for the bugs in the first place, because if you take a snapshot of a bug, that doesn't make it no longer a bug.