[BUG][Rust] nullable additionalProperties cannot be represented by model

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

In the latest version of the generator the following definition:

openapi: "3.0.0"
info:
  version: 1.0.0
  title: Bug
paths: {}
components:
  schemas:
    MyModel:
      type: object
      required:
      - my_field
      properties:
        my_field:
          type: object
          additionalProperties:
            type: string
            nullable: true

Will produce the following Rust model:

#[derive(Clone, Default, Debug, PartialEq, Serialize, Deserialize)]
pub struct MyModel {
    #[serde(rename = "my_field")]
    pub my_field: std::collections::HashMap<String, String>,
}

This fails to decode JSON which has additional null properties, for example:

{
    "my_field": {
        "extra_field": null
    }
}

The proper type to use for my_field would be:

HashMap<String, Option<String>>

I've created a possible fix for this problem in this commit.

Start with the OpenAPI definition and generated Rust model shown in the report, then inspect the linked commit for the proposed fix. Reproduce generation with a nullable additionalProperties value and verify that the resulting map accepts null values, using HashMap<String, Option> rather than HashMap<String, String>.