Updated IS_NULL filter to check for strict null.

The IS_NULL filter evaluates to true only when the
property is null. Updated tests to reflect this
change.

lib/OpenLayers/Filter/Comparison.js

@@ -156,7 +156,7 @@ OpenLayers.Filter.Comparison = OpenLayers.Class(OpenLayers.Filter, {
                 result = regexp.test(got);
                 break;
             case OpenLayers.Filter.Comparison.IS_NULL:
-                result = (got === null || got === undefined);
+                result = (got === null);
                 break;
         }
         return result;