Only get selection_name if it exist

  Idea: This should be improved to reuse proxyfied field method
        instead directly.

product/ERP5Form/ProxyField.py

@@ -762,12 +762,13 @@ class ProxyField(ZMIField):
       return proxy_field.get_value(id, **kw)
 
   def _cacheable_key(self):
-    selection_name = self.get_value('selection_name')
-    if selection_name is not None:
-      selection_tool = self.getPortalObject().portal_selections
-      selection = selection_tool.getSelectionFor(selection_name, REQUEST = self.REQUEST)
-      if selection is not None:
-        return ("list_start", selection.getParams().get("list_start"))
+    if self.has_value('selection_name'):
+      selection_name = self.get_value('selection_name')
+      if selection_name is not None:
+        selection_tool = self.getPortalObject().portal_selections
+        selection = selection_tool.getSelectionFor(selection_name, REQUEST = self.REQUEST)
+        if selection is not None:
+          return ("list_start", selection.getParams().get("list_start"))
     return "default"
 
   def _getCacheId(self):