The documentation reads:
====
The contents of the condition element is a jPDL expression that should evaluate to a boolean. A decision takes the first transition (as ordered in the processdefinition.xml) for which the expression resolves to true. If none of the conditions resolve to true, the default leaving transition (== the first one) will be taken.
====

If I have only a conditional transition in a node (hence the first one) it is not used as the default leaving transition.

Process definition to simulate the behaviour:
====
<?xml version="1.0" encoding="UTF-8"?>
<process-definition xmlns="urn:jbpm.org:jpdl-3.2" name="processDefinition3">
<start-state name="start">
<transition to="node1"></transition>
</start-state>

<node name="node1" async="true">
<transition to="node2" condition="#

"></transition>
</node>
<node name="node2">
<transition to="end1"></transition>
</node>
<end-state name="end1"></end-state>
</process-definition>
====

Exception being thrown on signaling process's token:
10:16:33,629 ERROR [JobExecutorThread] job execution failure
org.jbpm.JbpmException: transition is null
at org.jbpm.graph.def.Node.leave(Node.java:449)
at org.jbpm.graph.def.Node.leave(Node.java:430)
at org.jbpm.graph.def.Node.execute(Node.java:421)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:173)
at org.jbpm.graph.def.Node_$$javassist_81.execute(Node$$_javassist_81.java)
at org.jbpm.job.ExecuteNodeJob.execute(ExecuteNodeJob.java:24)
at org.jbpm.job.executor.JobExecutorThread.executeJob(JobExecutorThread.java:192)
at org.jbpm.job.executor.JobExecutorThread.run(JobExecutorThread.java:66)

JBPM-Console says:
Error signalling token: An exception of type "org.jbpm.JbpmException" was thrown. The message is: Node(node1) has no default transition