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.