MDEV-38853 Guard VALIDITY_ASSERT in Json_writer for NDEBUG builds

[alexDiaconuAlex]

This fixes a release/NDEBUG build failure caused by unguarded VALIDITY_ASSERT usage
in Json_writer::add_unquoted_str() and Json_writer::add_str().

The VALIDITY_ASSERT blocks are now compiled only for debug builds or when
JSON_WRITER_UNIT_TEST is enabled, matching the availability of the referenced symbols.

JIRA: MDEV-38853

[CLAassistant]

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3 out of 7 committers have signed the CLA.

✅ dbart
✅ hadeer-r
✅ alexDiaconuAlex
❌ Alexey Botchkov
❌ mariadb-poojalamba
❌ Thirunarayanan
❌ dr-m

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[grooverdan]

@alexDiaconuAlex , you appear to have taken @BjarneDMat's patches from JIRA and submitted them as your own without even an attribution to the author. Can you please credit the author git commit --author 'Bjarne D Mathiesen <email>' --amend or as a part of the commit message attribute the author.

10.6 is the earliest maintained version. Can you:

• Fix attribution
• Rebase commit to 10.6 branch
• force push to your github branch
• edit this PR near the title and change the target branch to 10.6

I note there's a collation of patches. If there are greater version numbers with an addition change compared to the 10.6 branch, create a PR for just that change against the right main MariaDB version.

[gkodinov]

Thank you for your contribution. This is a preliminary review.
The diff looks correct to me. As unfortunate that such #ifdefs are needed as it is.
Please (as Daniel suggests) re-base to 10.6 and also update the commit message to conform to CODING_GUIDELINES.md.

Then continue working with Daniel on the review.

Please also mention how exactly are you building the server in jira so it's documented.

[alexDiaconuAlex]

Thanks for the review! I've amended the commit message with proper attribution,
rebased the fix onto 10.6, force-pushed the branch, and retargeted the PR to 10.6.
Build steps were added to MDEV-38853.

I've also added the build steps to MDEV-38853 and documented the build method in jira.