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.

This situation can be triggered if the user clicks on the kfmclient_html.desktop file. In theory, this shouldn't happen; however, currently it's happening quite often, as can be seen, for example, from this bug report). As far as I could discover, this bug happens because the application launcher puts kfmclient_html.desktop in the "Favorites" section instead of konqbrowser.desktop. Clicking on it executes kfmclient openURL %u text/html, passing a literal %u as argument. Since %u isn't a valid URL, the behavior described above occurs.

While the correct fix for the bug itself is to find out why the application launcher uses kfmclient_html.desktop, I think that kfmclient should nonetheless have a more user-friendly behavior in case an invalid URL is given.