LPS-45541 Prevent NPE when field values don't contains _INSTANCE_

Hey Marcellus, we found this error after your commit for DDMImpl: http://ci.liferay.org.es/jenkins/job/mdelapenya/579/testReport. It throws a NPE when fields are not well-formed. I've added _INSTANCE_ suffix to field generation, but... won't be worth it adding that cappability to the DDM impl instead of clients knowing the internals of DDM? What do you think? Thanks for your time reviewing this buddy\!