Ticket 23829

This pull requests fixes the actual problem with #23829 and also makes the command work if django.contrib.sites is not installed.

`django.contrib.sites` will default to the current requests to get the domain, but this is not applicable when running as a management command.

I could not find any existing tests for this, so I did not know which tests to enhance. However, I will work on writing some mocks to ensure that the request is being passed correctly.
Indentation is a bit weird here.

```rst
 .. function:: ping_google

     :func:`ping_google` takes the following optional arguments:

     * ``sitemap_url``, which [...]
```

looks better to me.