Improve behavior when kfmclient openURL is launched with invalid URL
If kfmclient openURL is launched with an invalid URL, filteredURL will return an empty URL which causes Konqueror to display an "Undocumented error" error message. To avoid this, when filteredUrl returns an empty URL, use the user's home URL, as if no URL had been specified. In this case we also ignore the mimetype requested by the user (if any) because in most cases it won't be correct for the new URL.