Add missing type mapping

Fixes DDC-2494
Refs DDC-3192

This is essentially a fix from DDC-2494 applied to SQLWalker.
The issue: type was not converted to PHP value when the result is fetched by
executing DQL query rather than using entity manager's findX(). Similar issue
for BasicEntityPersister (which is used when em's findX is executed) was fixed
in DDC-2494, but SQLWalker made the issue valid for any custom query.