fix Calling get(0) without checking the event definitions of targetEvent

[panshaobin]

fix Calling get(0) without checking the event definitions of targetEvent

[filiphr]

@panshaobin if this can happen then please provide a test for this. We have

 if (sourceEvent.getEventDefinitions() != null && !sourceEvent.getEventDefinitions().isEmpty() && 
        (targetEvent.getEventDefinitions() == null || targetEvent.getEventDefinitions().isEmpty())) {

    return false;

}

in the beginning of the method. So if the source event definitions are null or empty and the target event definitions are null or empty then we are going to return earlier, which means at the place you added the target event definitions cannot be null or empty