修复流程节点为空时的处理逻辑

src/main/java/com/lanyuanxiaoyao/flowable/model/Flow.java

@@ -74,7 +74,7 @@ public class Flow {
    */
   public void addNode(FlowNode node) {
     nodes.add(node);
-    if (currentNode == null && !nodes.isEmpty()) {
+    if (currentNode == null) {
       currentNode = nodes.get(0).getNodeId();
     }
   }