Treat SQL null as null when result set mapping

Currently a null integer column maps to 0 rather than null making it impossible to distinguish SQL nulls. This change maps SQL null to Java null in AnnoMapper.get() which actually just means the setter is never called by AnnoMember.write().

Note that this is potentially a breaking change if someone relied on the null to 0 mapping behaviour.

I used a public field rather than getter and setter methods for the "value" column in the test so we get some test coverage of both access styles.