Add note on using None for empty strings
This has come up in a few issues and maybe this will head that off.
@@ -392,7 +392,8 @@ the ``FuzzyCategorical`` Type. For more info, see the `GitHub Repository
Missing Data
------------
If the value of field is missing, that missing value should be represented as
-a ``None`` object.
+a ``None`` object. You should also use ``None`` to represent empty strings
+(eg ``''``).
.. code:: python
