public Boolean execute(ComplexEvent event) {
        if (eventPosition == null) {
            if (event == null) {
                return Boolean.TRUE;
            } else {
                return Boolean.FALSE;
            }
        } else {
            StreamEvent streamEvent = ((StateEvent) event).getStreamEvent(eventPosition);
            if (streamEvent == null) {
                return Boolean.TRUE;
            } else {
                return Boolean.FALSE;
            }
        }
    }
Executor class for IsNullStream condition. Condition evaluation logic is implemented within executor.
modules/siddhi-core/src/main/java/io/siddhi/core/executor/condition/IsNullStreamConditionExpressionExecutor.java