DxBuilder: Fix check if field is present (to determine if default values should be set).

(field.get() *will* raise an AttributeError if the field doesn't exist on obj, and
can't be provided with a default value)
Merge pull request #25 from 4teamwork/lg-fix-check-for-field-presence

DxBuilder: Fix check if field is present (to determine if default values should be set).