Use Autodiscovery when Proxy URL is blank

Right now you cannot Apply proxy settings when the "URL to configuration script" is blank, as it is treated as a required field.

Instead, the network plug should use proxy autodiscovery when the "URL to configuration script" is blank. This is consistent with how the GNOME plug handles proxies and is required for networks that rely on autodiscovery.

Launchpad Details: [#LP1620700](https://bugs.launchpad.net/bugs/1620700) Cassidy James Blaede - 2016-09-06 16:13:38 +0000

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