support overloading a related key by nothing.

use "related_key_name | " as key definition
(watch out for whitespace preserve in the field)

product/ZSQLCatalog/SQLCatalog.py

@@ -2041,8 +2041,10 @@ class Catalog(Folder,
           LOG('SQLCatalog', WARNING, 'Malformed related key definition: %r. Ignored.' % (entire_definition, ))
           continue
         if split_entire_definition[0].strip() == key:
-          result = split_entire_definition[1].strip()
-          break
+          key_definition = split_entire_definition[1].strip()
+          if key_definition:
+            result = key_definition
+            break
       else:
         result = None
       related_key_definition_cache[key] = result