ZSQLCatalog: Also cache misses.

There are a lot of misses in this function because related keys must have
precedence over homonym columns, which makes it cost a *lot* of cpu time
to just return None.

product/ZSQLCatalog/SQLCatalog.py

@@ -2035,7 +2035,6 @@ class Catalog(Folder,
     try:
       result = related_key_definition_cache[key]
     except KeyError:
-      result = None
       for entire_definition in self.getSQLCatalogRelatedKeyList([key]):
         split_entire_definition = entire_definition.split('|')
         if len(split_entire_definition) != 2:
@@ -2044,7 +2043,8 @@ class Catalog(Folder,
         if split_entire_definition[0].strip() == key:
           result = split_entire_definition[1].strip()
           break
-      if result is not None:
+      else:
+        result = None
       related_key_definition_cache[key] = result
     return result