store/internal: validate keys before calling ProofsFromMap (backport #9235)

[mergify]

This is an automatic backport of pull request #9235 done by Mergify.
Cherry-pick of 711976e has failed:

On branch mergify/bp/release/v0.42.x/pr-9235
Your branch is up to date with 'origin/release/v0.42.x'.

You are currently cherry-picking commit 711976efc.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   store/internal/proofs/create.go
	modified:   store/internal/proofs/create_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md

To fix up this pull request, you can check it out locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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