UI: Show message in transform panel when no metaball elements are selected

The complete lack of a transform panel was confusing enough to spawn
this question: https://blender.stackexchange.com/q/169074/599
Displaying a message instead of nothing is more consistent with the
behavior of the mesh transform panel anyway.

Reviewed By: Blendify

Differential Revision: https://developer.blender.org/D8390

source/blender/editors/space_view3d/view3d_buttons.c

@@ -1474,6 +1474,7 @@ static void v3d_editmetaball_buts(uiLayout *layout, Object *ob)
   uiLayout *col;
 
   if (!mball || !(mball->lastelem)) {
+    uiItemL(layout, IFACE_("Nothing selected"), ICON_NONE);
     return;
   }