[5.x]: Handle NULL string attributes in HDF5.

[lesserwhirls]

Description of Changes

Port #798 from develop to 5.x, for both old and new APIs. Closes #797.

PR Checklist

• Indicate the version associated with this PR in the Title
  (e.g. "[5.x]: This is my PR title")
• Link to any issues that the PR addresses
• Add labels, especially if the PR should be ported to other versions
  (these labels start with "port: ")
• Open as a draft PR
  until ready for review
• Make sure GitHub tests pass
• Mark PR as "Ready for Review"

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This change is 

[rschmunk]

Oh, good, this is the push I was waiting for.

[haileyajohnson]

Reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @lesserwhirls)