CMFActivity.Activity.SQLBase: Fix None dependency on monocolumn condition.

This is already working for multi-column conditions (among other malformed
values), but in the monocolumn case the type-check treats None in the same
branch as sequences, which cause the error:
  TypeError: 'NoneType' object is not iterable
As the condition is already covered for multi-column case, put this test
in the monocolumn codepath.

product/CMFActivity/Activity/SQLBase.py

@@ -454,6 +454,8 @@ CREATE TABLE %s (
         # enumerate all possible combinations for later reverse-lookup.
         column_count = len(column_list)
         if column_count == 1:
+          if dependency_value is None:
+            continue
           dependency_value_list = [
             x
             for x in (