Don't append a null if we're clearing out a local attribute that's not set.

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@5528 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/java/com/threerings/presents/dobj/DObject.java

@@ -651,7 +651,12 @@ public class DObject
             }
         }
 
-        // append our attribute to the end of the list
+        // if we were trying to clear out an attribute but didn't find it, then stop here
+        if (attr == null) {
+            return;
+        }
+
+        // otherwise append our attribute to the end of the list
         _locattrs = ArrayUtil.append(_locattrs, attr);
     }