[FLOSS H3] toml: fix silent failure on meta-key insertion

[Bujuhu]

Fixes #4579

Basics

• Short descriptions of your changes are in the release notes
  (added as entry in doc/news/_preparation_next_release.md which
  contains _(my name)_)
  Please always add something to the release notes.
• Details of what you changed are in commit messages
  (first line should have module: short statement syntax)
• References to issues, e.g. close #X, are in the commit messages.
• The buildservers are happy. If not, fix in this order:
  • add a line in doc/news/_preparation_next_release.md
  • reformat the code with scripts/dev/reformat-all
  • make all unit tests pass
  • fix all memleaks
• The PR is rebased with current master.

Checklist

• I added unit tests for my code
• I fully described what my PR does in the documentation
  (not in the PR description)
• I fixed all affected documentation (see Documentation Guidelines)
• I fixed all affected decisions (see Decision Process)
• I added code comments, logging, and assertions as appropriate (see Coding Guidelines)
• I updated all meta data (e.g. README.md of plugins and METADATA.ini)
• I mentioned every code not directly written by me in reuse syntax

Review

• Documentation is introductory, concise, good to read and describes everything what the PR does
• Examples are well chosen and understandable
• Code is conforming to our Coding Guidelines
• APIs are conforming to our Design Guidelines
• Code is consistent to our Design Decisions

Labels

• Add the "work in progress" label if you do not want the PR to be reviewed yet.
• Add the "ready to merge" label if the basics are fulfilled and no further pushes are planned by you.

[Dynamichost96]

Documentation is introductory, concise, good to read and describes everything what the PR does

All good!

Examples are well chosen and understandable
N/A

Code is conforming to our Coding Guidelines
Code is conforming to the Guidelines

APIs are conforming to our Design Guidelines
N/A

Code is consistent to our Design Decisions
Code is consistent to the Guidelines

[markus2330]

Great job! ❤️