Support openapi style JSON schema output

[robaca]

In OpenAPI 3 documents, "type": ["null", "string"] is not supported. Instead you have to specify a property via "type": "string", "nullable": true"

Please provide a command line argument to optimize the output for OpenAPI 3 compatibility.

[domoritz]

As far as I know, nullable is not a valid JSON schema property. Unless I am missing something, it is not possible to do this, unfortunately.