don't assume property is not nullable when no "nullable" field on property

[nbushak]

Most of the time, the properties set on SQLAlchemy models are the Column datatype. These are guaranteed to have a nullable attribute set of either True or False:
https://github.com/zzzeek/sqlalchemy/blob/master/lib/sqlalchemy/sql/schema.py#L1210

I tested this by print()ing the value of .nullable for all my properties in is_column_nullable(). All were set, and always True or False.

However, column_property() properties do not have a nullable attribute set. It's incorrect to assume these properties are not nullable. Here is an example, which returns null and throws in graphene when func.count(PersonInfo.id) is 0:

Reachout.replied_percent = column_property(
    select([
        func.sum(func.cast(PersonInfo.has_replied, Integer)) /\
            func.cast(func.count(PersonInfo.id), Float)
    ]).where(
        PersonInfo.reachout_id == Reachout.id,
    ),
)

[coveralls]

Coverage remained the same at 91.129% when pulling 2c5b103 on nbushak:master into c1a0cae on graphql-python:master.