Issue title: [BUG] [typescript-angular]: objects as query parameters break when fields are null
 Issue body: #### Bug Report Checklist

- [x] Have you provided a full/minimal spec to reproduce the issue?
- [X] Have you validated the input using an OpenAPI validator ([example](https://apidevtools.org/swagger-parser/online/))?
- [X] What's the version of OpenAPI Generator used?
- [X] Have you search for related issues/PRs?
- [X] What's the actual output vs expected output?

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##### Description

<!-- describe what is the question, suggestion or issue and why this is a problem for you. -->

When using the "objects as query parameter" support introduced in #4407, NULL fields cause an exception `TypeError: Cannot convert undefined or null to object` in the `addToHttpParamsRecursive` function of the generated service.

##### openapi-generator version

4.2.3

##### OpenAPI declaration file content or url

```yaml
openapi: 3.0.1
info:
  title: Test API
  version: v0.0.1
servers:
- url: http://localhost:8080
paths:
  "/myobjects/filtered":
    get:
      operationId: findAllByFilter
      parameters:
      - name: filter
        in: query
        required: true
        schema:
          "$ref": "#/components/schemas/MyObjectFilter"
      responses:
        '200':
          description: default response
          content:
            "*/*":
              schema:
                type: "string"
components:
  schemas:
    MyObjectFilter:
      type: object
      properties:
        query:
          type: string
        type:
          type: string
          enum:
          - TYPE1
          - TYPE2
```

##### Command line used for generation
```shell script
openapi-generator generate -g typescript-angular -o .generated -i .swagger-spec.json
```

##### Steps to reproduce

1. Generate the code and copy it to an Angular application
2. Create a new MyObjectFilter with `query==null`:
```typescript
const filter = {
  query: null,
  type: "TYPE1"
};
```
3. Try to use the service:
```typescript
this.myObjectService.findAllByFilter(myObjectFilter)
```

##### Related issues/PRs

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None found.

##### Suggest a fix

<!-- if you can't fix the bug yourself, perhaps you can point to what might be
  causing the problem (line of code or commit), or simply make a suggestion -->

I have found that adding a NULL-check in the generated addToHttpParamsRecursive method seems to fix the issue:
```typescript
private addToHttpParamsRecursive(httpParams: HttpParams, value: any, key?: string): HttpParams {
  if(value == null) {
    return httpParams;
  }
  // Original code starts here...
}
```
 Comments: 
 Comment 0: @daniel-frak would you like to file a PR to fix this issue?
 Comment 1: cc @aanno2 
 Comment 2: I've filed #5197, let me know if I did this right :)
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