title: HttpParams not send param with blank string
body: <p>I have this kind of code, where I'm requesting an API with an empty string for the query param:</p>
  
  <pre><code>  getArticles(params?: HttpParams): Observable&lt;any&gt; {
      return this.http.get(this._articlesUrl, {
        params:  new HttpParams()
          .set('page', this.page.toString())
          .set('per_page', this.limit.toString())
          .set('query', this.query.toString())
      });
    }
  </code></pre>
  
  <p>After calling the <strong>getArticles</strong> function, I'm getting this king of url: <code>127.0.0.1:8000/api/articles?page=1&amp;per_page=6&amp;query=</code> <br>
  The question is: Does there any clean way where the <code>query</code> param will not appear in the url, if it's an empty string ?</p>