status/network: Ensure the Cancel button passes undefined to close()

status/network: Ensure the Cancel button passes undefined to close()

The parameter to `ModalDialog.close(timestamp)` is optional. But when
invoked via the network dialog's Cancel button it was receiving an
implicit parameter value that's definitely not a timestamp:

```
[0x560f18af0c50 StButton.modal-dialog-linked-button:first-child hover ("Cancel")]
```

And as of today (or gjs >= 1.67.3) that's reported as an error:
```
JS ERROR: Error: Argument timestamp: value is out of range for uint32
popModal@resource:///org/gnome/shell/ui/main.js:638:12
popModal@resource:///org/gnome/shell/ui/modalDialog.js:206:14
close@resource:///org/gnome/shell/ui/modalDialog.js:179:14
```
and so you can't Cancel the dialog anymore.

Make sure `ModalDialog.close()` receives an `undefined` timestamp it
knows how to handle.

Fixes: https://bugs.launchpad.net/bugs/1918666
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1757>