Minimal does not set response type nullability

[mikekistler]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

If a minimal API app returns null / sets the response body to null, the response body sent on the wire is "null", but the generated OpenAPI does not indicate that the response body is nullable.

Expected Behavior

The response body type should specify nullable: true if the response could be "null".

Steps To Reproduce

No response

Exceptions (if any)

No response

.NET Version

9.0.100-rc.1.24452.12

Anything else?

No response

[captainsafia]

Let's try to do this for RC1 as part of the nullability reactions.

[captainsafia]

Closing out as a dupe of #63029.