fix(datamapper): returns the value if it is considered empty

Using checker.isEmpty null, undefined, {} or "" values will be returned by themselves, without being
converted to null or undefined and without being passed to the parser.

BREAKING CHANGE: This can change the way a value that does not exist in the collection is
undefined in the application that uses the lib.
A null value is returned as null, but an
undefined is passed to the parser and can return a different value.
Instead of "undefined",
undefined will be returned, causing breaking changes in systems that expect "undefined" with a
string type.

herbsjs#36 and herbsjs#39

src/dataMapper.js

@@ -143,7 +143,7 @@ class DataMapper {
       }
 
       function dataParser(value, parser) {
-        if (value === null) return null
+        if (checker.isEmpty(value)) return value
         return parser(value)
       }