special case empty object for jtd

[erikbrinkman]

What issue does this pull request resolve?

fixes #2123

What changes did you make?

In short, this special cases empty objects since they have special meaning due to the way typescripts type system works. I'll leave more detail in the issue.

Is there anything that requires more attention while reviewing?

I did some various "clean up" to JTD Schema as well, but the substantiative change is special casing that type and adding tests. I think the test coverage is pretty solid (although clearly not perfect) so I wouldn't be too worries.

[epoberezkin]

Great, thank you!

[epoberezkin]

It failed when merging master in - will check later what happened…

[erikbrinkman]

The build failure is

Error: node_modules/re2/re2.d.ts(13,13): error TS24[30](https://github.com/ajv-validator/ajv/actions/runs/3507783326/jobs/5875718510#step:4:31): Interface 'RE2' incorrectly extends interface 'RegExp'.
  The types returned by 'exec(...)' are incompatible between these types.
    Type 'RE2ExecArray<string> | null' is not assignable to type 'RegExpExecArray | null'.
      Property '0' is missing in type 'RE2ExecArray<string>' but required in type 'RegExpExecArray'.

which seems unrelated to this merge. Seems like a versioning issue, but probably worth investigating :/