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

[PaulWagener]

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.