Fixed super-class iteration for six.with_metaclass decoration in Django 1.5 (PY-8137).

python/psi-api/src/com/jetbrains/python/psi/PyClassRef.java

@@ -14,12 +14,12 @@ public class PyClassRef {
   @Nullable
   private final String myQName;
 
-  public PyClassRef(PsiElement element) {
+  public PyClassRef(@Nullable PsiElement element) {
     myElement = element;
     myQName = null;
   }
 
-  public PyClassRef(String qName) {
+  public PyClassRef(@Nullable String qName) {
     myElement = null;
     myQName = qName;
   }