Cacerts store, before Java 10 is an empty set. It's needed to have a group of certificates that may be wont to initiate trust in certificates chain of varied security protocols of vendors.
OpenJDK builds don't have such certificates that are why critical security elements such as TLS didn't add default builds.
Now as Oracle has started sourcing the primary certificates by making use of the Oracle JAVA SE Root CA program, OpenJDK builds might now have root certificates and thus can decrease the variation between OpenJDK and Oracle JDK.
Oracle JAVA SE Root CA program issues the basis certificates. Vendors who've signed the agreement, are included within the set of root certificates. The vendors who aren't registered are going to be included in the next launch.