Prevent column summation if no non-None cells

Change-Id: I96ec5f5c6391c9234f79ad751b5c1671eda393ef
Use default_if_none rather than default filter

Zero is a valid summation for a column, so only default to
a hyphen when a summation is explicitly None.

Change-Id: Ib523657ca46f9f2563ddb82cb2a3b64d9458979d