Make QueryEntity.keyFields optional for caches having SQL types as keys

Initial implementation of DML requires the user to specify keyFields in configuration explicitly, even if it's empty, in cases when DML is used and binary marshaller is used. As vozerov noted, it could affect usability, if even a little, so this should be fixed - when a primitive/simple SQL type is used as a key, it's obvious that there's no key fields and hence it's not necessary to specify empty keyFields in this case.