UI Tweak: "Add to Group" button creates new group if no groups exist

Previously, the "Add to Group" button would show an empty search popup when there were no existing groups. While this does mean that the button behaves differently at different times, this way is more streamlined and should be less confusing than seeing an empty search popup or a greyed out "add to group" button or a "+" button which jumps around in different situations.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55435 954f8c5b-7b00-dc11-b283-0030488c597c