Added support for the CPNullPlaceholderBindingOption for bound text fields.

AppKit/CPObjectController.j

@@ -663,7 +663,7 @@ var CPObjectControllerContentKey                        = @"CPObjectControllerCo
 
 - (id)_controllerMarkerForValues:(CPArray)theValues
 {
- var count = [theValues count];
+    var count = [theValues count];
 
     if (!count)
         value = CPNoSelectionMarker;
@@ -685,6 +685,9 @@ var CPObjectControllerContentKey                        = @"CPObjectControllerCo
         }
     }
 
+    if (value === nil || value.isa && [value isEqual:[CPNull null]])
+        value = CPNullMarker;
+
     return value;
 }