Don't try to restore invalid user places

Summary:
Don't restore user places with invalid attribute href.
This avoids asking for empty protocol in KFilePlacesItem::groupType and the assert in KProtocolInfoFactory::findProtocol.

Test Plan:
Don't assert asking for invalid protocols when creating the save dialog with an invalid bookmark (empty href) in ~/.local/share/user-places.xbel

<bookmark href="">
 <title>Project Folder</title>
 <info>
  <metadata owner="http://freedesktop.org">
   <bookmark:icon name="folder-favorites"/>
  </metadata>
  <metadata owner="http://www.kde.org">
   <OnlyInApp>kdenlive</OnlyInApp>
  </metadata>
 </info>
</bookmark>

Reviewers: dfaure, Frameworks

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14072