[Bug]: Nullability problems with dictionaries

[ozziepeeps]

Describe the bug

Two issues: Dictionary<string, int>? in a nullable context is incorrectly described with the int value being nullable. Additionally the behavior of Swashbuckle changes if my property type is IDictionary<string, int>? vs Dictionary<string, int>? (e.g. interface vs concrete).

Expected behavior

If I have a property nullableMapNonNullableValue of type Dictionary<string, int>? or IDictionary<string, int>? on a schema, I would expect the int value to be described as NOT nullable in additionalProperties, like so:

{
    "nullableMapNonNullableValue": {
        "type": "object",
        "additionalProperties": {
            "type": "integer",
            "format": "int32"
        },
        "nullable": true
    }
}

Actual behavior

The int value in additionalProperties is incorrectly described as being nullable:

{
    "nullableMapNonNullableValue": {
        "type": "object",
        "additionalProperties": {
            "type": "integer",
            "format": "int32",
            "nullable": true // <-- this is wrong!
        },
        "nullable": true
    }
}

If, however, I change the type of the property to be IDictionary<string, int>? (e.g. interface instead of concrete) the behavior changes and the nullability of the int value is now described correctly in additionalProperties:

{
    "nullableMapNonNullableValue": {
        "type": "object",
        "additionalProperties": {
            "type": "integer",
            "format": "int32"
        },
        "nullable": true
    }
}

Steps to reproduce

I'm submitting a PR with unit tests that fixes both issues.

Exception(s) (if any)

No response

Swashbuckle.AspNetCore version

6.7.0

.NET Version

net8.0

Anything else?

No response