Type tweaks

[entropi]

This has a couple small tweaks:

• things like Enums don't have properties in their definition, so wrapped the property logic in a key check
• for anyOf types that can't be handled simply, set the type as string so that it's not entirely missing a type.
• allow for the case where there are zero security definitions, as mentioned in https://github.com/virajkanwade/fastapi_swagger2/issues/2