[rust client] Incomplete type in response generates code that fails to compile

Description

The Rust generated code fails to compile if the api defines a response type of object with no additional information. Other code generators like C# do generate code that compiles and runs even if it isn't a very useful response.

The errors look like:

error[E0412]: cannot find type `Value` in this scope
  --> src\apis\file_api.rs:34:44
   |
34 |     fn clone(&self, ) -> Box<Future<Item = Value, Error = Error<serde_json::Value>>>;
   |                                            ^^^^^ not found in this scope
help: possible candidates are found in other modules, you can import them into scope
   |
11 | use serde_json::Value;
   |
11 | use serde_json::value::Value;
   |

error[E0412]: cannot find type `Value` in this scope
  --> src\apis\file_api.rs:39:44
   |
39 |     fn clone(&self, ) -> Box<Future<Item = Value, Error = Error<serde_json::Value>>> {
   |                                            ^^^^^ not found in this scope
help: possible candidates are found in other modules, you can import them into scope
   |
11 | use serde_json::Value;
   |
11 | use serde_json::value::Value;

openapi-generator version

3.3.2

OpenAPI declaration file content or url

{
  "swagger": "2.0",
  "info": {
    "version": "v1.0",
    "title": "test api"
  },
  "paths": {
    "/api/clone": {
      "put": {
        "tags": [
          "File"
        ],
        "operationId": "Clone",
        "consumes": [],
        "produces": [
          "application/json",
          "text/json"
        ],
        "parameters": [],
        "responses": {
          "200": {
            "description": "OK",
            "schema": {
              "type": "object"
            }
          }
        }
      }
    }
  }
}

Command line used for generation

java -jar openapi-generator-cli.jar generate -i api.json -g rust

Steps to reproduce

java -jar openapi-generator-cli.jar generate -i api.json -g rust
cargo build

Related issues/PRs

Suggest a fix/enhancement

It looks like the return type could be serde_json::Value instead of Value, maybe as simple as changing this line.

Start with RustClientCodegen.java at the linked line and inspect how an empty object response is mapped. Reproduce the issue with the provided OpenAPI document and command, then run cargo build on the generated client. Done means the generated Rust code compiles for this response type.