NetworkLink not always contains an href, fixing #1159

@Kartones , take a look. I don't know much about KML, but the issue had a file that had NetworkLinkControl (https://developers.google.com/kml/documentation/kmlreference#networklinkcontrol) tag that doesn't contains any actual link, and can be imported straightforward. I thought it's cleaner searching for href than excluding NetworkLinkControl from NetworkLink (https://developers.google.com/kml/documentation/kmlreference#networklink) match.