Setting ErrorRef to blank for an error start event works on jbpm 6.4.
attached project to understand issue in detail.
Issue can be replicated on JBDS 12.9 and Code Ready 12.12 with jbpm plugin installed.(Red Hat Business Process and Rules Development:plugin)

To replicate the issue follow the below steps:
1.Import project into business-central>Build & Deploy>Start Main Process from Process instance(this works as expected).
2.Import project into jbds/code-ready build to kjar >deploy on business-central>start process>even this works.
3.The moment we try to create an event sub-process equivalent to MainProcess,there are compilation issues of the process on JBDS/Code-Ready.
Msg:Error Error Code has no Data Type ,Output from Error Event definition is not mapped to a target data item.