Allow applications to force nullability of columns

[ajcvickers]

Split off from #13850

By default, a column is nullable in the database if the property to which it is mapped is nullable. This is true even if the converter itself never converts nulls, since by default nulls are passed through unconverted. However, if the value converter converts nulls and ensures that no null values result from this conversion, then it it can make sense for the database column to be non-nullable.