[BUG][Rust] Optional query parameters are always sent

Created by: jonahgeorge

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

When generating a Rust client with the Reqwest library, optional query parameters are not generated as Option<T> and are always sent in the request. This is an issue for APIs which treat an empty string differently from presence of field.

openapi-generator version

4.0.0

OpenAPI declaration file content or url

---
openapi: 3.0.0
info:
  title: Bug Example
  version: 1.0.0
paths:
  /users:
    get:
      parameters:
        - in: query
          name: given_name
          schema:
            type: string
        - in: query
          name: family_name
          required: true
          schema:
            type: string
      responses:
        "200":
          description: ""
          content:
            application/json:
              schema:
                type: object

Command line used for generation

openapi-generator generate -g rust -i openapi.yaml --library reqwest -o actual/

Expected Output

impl DefaultApi for DefaultApiClient {
    fn users_get(&self, family_name: &str, given_name: Option<&str>) -> Result<Value, Error> {
        let configuration: &configuration::Configuration = self.configuration.borrow();
        let client = &configuration.client;

        let uri_str = format!("{}/users", configuration.base_path);
        let mut req_builder = client.get(uri_str.as_str());

        if let Some(ref s) = given_name {
            req_builder = req_builder.query(&[("given_name", &s.to_string())]);
        }
        req_builder = req_builder.query(&[("family_name", &family_name.to_string())]);

Actual Output

impl DefaultApi for DefaultApiClient {
    fn users_get(&self, family_name: &str, given_name: &str) -> Result<Value, Error> {
        let configuration: &configuration::Configuration = self.configuration.borrow();
        let client = &configuration.client;

        let uri_str = format!("{}/users", configuration.base_path);
        let mut req_builder = client.get(uri_str.as_str());

        req_builder = req_builder.query(&[("given_name", &given_name.to_string())]);
        req_builder = req_builder.query(&[("family_name", &family_name.to_string())]);

Suggest a fix

#3053