[html2] Add oneOf support.

[Aniokrait]

fix #13313
The oneOf was not supported because of typo.
Also, I added the null check for schema.properties because oneOf doesn't have properties.

How to validate the work

Use minimal reproducable spec of #13313 (here)

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[wing328]

thanks for the PR, which has been merged.

when you've time next week, can you please PM me via Slack when you've time? https://join.slack.com/t/openapi-generator/shared_invite/zt-12jxxd7p2-XUeQM~4pzsU9x~eGLQqX2g

have a nice weekend