allow invalid bindings in disabled groups

This fixes a regression introduced in
7f83ff024e1fe56c71d193a95cc9af988c12660d.

It's now valid again to do something like
Group {
   condition: qbs.targetOS.contains("haiku")
   cpp.specialHaikuOption: "yes, please"
}

Task-number: QBS-409

Change-Id: I693df9202603c9af7fd7482fa537ed1fb16591a7
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>