Handle untyped schemas

[gibson042]

Fixes #6896 Depends upon https://github.com/swagger-api/swagger-parser/pull/573

PR checklist

• [x] Read the contribution guidelines.
• [ ] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• [x] Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• [ ] Copied the technical committee to review the pull request if your PR is targeting a particular programming langauge.

Description of the PR

As a foundation for #6896, treat untyped schemas like type: object (since many languages have a root type, and even those that don't are fairly well-behaved here) and save language-specific work for later.

[wing328]

@gibson042 thanks for the PR. We'll wait for https://github.com/swagger-api/swagger-parser/pull/573 to be merged first.

[3ygun]

@wing328 any update on this one? Seems to be the root problem of (only the Java client ones):

• #6896
• #7565
• #7586

[gibson042]

@wing328 This should now be good to go.

[wing328]

@gibson042 thanks for tagging me but I'm no longer involved in this project. I hope others will be able to help you out. Good luck.