When deploying a process definition that has a conditional event (intermediate or boundary) which has no expression set, the engine throws an error when trying to start an instance of the process.
This is expected.

We should check this when deploying the process definition and prevent the deployment as it can never be executed completely.
See org.camunda.bpm.engine.impl.bpmn.parser.BpmnParse

Steps to reproduce:

deploy attached bpmn file -> no error
start instance -> error
ENGINE-16004 Exception while closing command context: condition expression returns non-Boolean: result has class java.lang.String and not java.lang.Boolean