Error: "nullable" cannot be used without "type" for properties without types

[dgmora]

This is similar to #228, but I am having this error for a property without type:

properties:
  name:
    nullable: true

There's this issue that suggests that it is not required (a few other tools referenced this issue). The issue in my specific case is that the openapi spec is generated by a tool that cannot infer the type. It would be nice if openapi-backend could handle this (apparently valid) case.