How to set JSON response_format for Gemini models if a property can have multiple types?

[Nickk4]

I'm trying to get OpenAI and Gemini models to use the same json structure for response. My response format for OpenAI models use strict: true but for Gemini I leave this out, since they don't accept that property.

The response format includes:

abc: {
  type: ['string', 'null'],
  description: 'Your answer to Question 3 goes here. Leave as null if no information is available.',
},

This works with OpenAI models but Gemini does not seem to accept a type of ['string', 'null'].
I've tried using oneOf as an alternative, but Gemini also doesn't seem to support that.

I could do something like:

abc: {
  type: 'string',
  description: 'Your answer to Question 3 goes here. Enter null if no information is available.',
},

But then the LLM will probably just include a null string instead of a real null value. And more importantly, it's not aligned with OpenAI's strict JSON structure.

What is the best way to deal with this?
(Is there really no better option than to define separate response structures for Gemini/OpenAI?)