Update comment to reflect that the property can return null as well as undefined.

src/Mesh/babylon.transformNode.ts

@@ -156,7 +156,7 @@ module BABYLON {
         }
 
         /**
-         * Gets or sets the rotation Quaternion property : this a Quaternion object defining the node rotation by using a unit quaternion (null by default).
+         * Gets or sets the rotation Quaternion property : this a Quaternion object defining the node rotation by using a unit quaternion (undefined by default, but can be null).
          * If set, only the rotationQuaternion is then used to compute the node rotation (ie. node.rotation will be ignored)
          */
         public get rotationQuaternion(): Nullable<Quaternion> {