Avoid KeyError when attempting to access "properties" key

[larsks]

Closes #54

[spiarh]

hi @garethr , any chance to see this merged ? This fixed the issue we hit running this script against the openapi resources from OpenShift 4.8. This allowed the script to generate the shared definitions.

Thanks @larsks and @garethr

[ReDestroyDeR]

Hello @garethr, up on this PR. Issue #54 got a response from me and @paulcwarren and this commit is fixing it. When it's getting merged?

Thanks.

[bilbof]

I hit this issue with OpenShift 4.8 - can confirm the fix in the commit resolves it, thanks @larsks 👍

[larsks]

I'm collecting fixes in https://github.com/larsks/openapi2jsonschema. I've merged this patch into that repository.