R4R: Assert store implementations never store nil values

[cwgoes]

Closes #2707

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added entries in PENDING.md with issue #
• rereviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #3435 into develop will increase coverage by 0.03%.
The diff coverage is 84.61%.

@@             Coverage Diff             @@
##           develop    #3435      +/-   ##
===========================================
+ Coverage    59.36%   59.39%   +0.03%     
===========================================
  Files          134      135       +1     
  Lines         9972     9975       +3     
===========================================
+ Hits          5920     5925       +5     
+ Misses        3679     3678       -1     
+ Partials       373      372       -1