title: pass value as empty string to WEb API from typescript
body: <p>I want to pass an empty string to C# web api from javascript.
  But when i pass empty string from typescript i get null in the webapi parameter.
  How to pass empty string?</p>
  
  <p>here is my client web api call:</p>
  
  <pre><code>public ChangePassword(username: string, oldPassword: string, newPassword: string) {
      const oldPwd = (oldPassword === null || oldPassword === undefined) ? '' : oldPassword;
      const newPwd = (newPassword === null || newPassword === undefined) ? '' : newPassword;
  
      const endPointUrl: string = this.webApi.EndPoint + '/Authentication/ChangePassword';
      const parameters = new HttpParams()
          .set('username', username)
          .set('oldPassword', oldPwd)
          .set('newPassword', newPwd);
      return this.httpClient.post(endPointUrl, '', { params: parameters });
  }
  </code></pre>
  
  <p>And my web api is </p>
  
  <pre><code>[HttpPost]
      public void ChangePassword(string userName, string oldPassword, string newPassword)
      {
          WebServiceFault fault = _securityManager.ChangePassword(userName, oldPassword, newPassword);
          if (fault == null)
          {
              return;
          }
  
          throw WebApiServiceFaultHelper.CreateFaultException(fault);
      }
  </code></pre>
  
  <p>When i pass null as parameter for old and new password to the <code>ChangePassword()</code> method i get the string as null in the web api method insted of getting an empty string.</p>