fix: Custom cell render fails with plain string

[GordonSmith]

Cell of width zero should never render.

Checklist:

• [ ] The commit message is properly formatted and free of typos.
  • [ ] The commit message title makes sense in a changelog, by itself.
  • [ ] The commit message includes a "fixes" reference if appropriate.
  • [x] The commit is signed.
• [x] The change has been fully tested:
  • [ ] I have viewed all related gallery items
  • [ ] I have viewed all related dermatology items
• [x] There are no similar instances of the same problem that should be addressed
  • [ ] I have addressed them here
  • [ ] I have raised new issues to address them separately

Testing: