Fix dependency failure (#5239)

Co-authored-by: JinyLeeChina <297062848@qq.com>

dolphinscheduler-common/src/main/java/org/apache/dolphinscheduler/common/model/TaskNode.java

@@ -211,8 +211,10 @@ public class TaskNode {
     }
 
     public void setDepList(List<String> depList) {
-        this.depList = depList;
+        if (depList != null) {
-        this.preTasks = JSONUtils.toJsonString(depList);
+            this.depList = depList;
+            this.preTasks = JSONUtils.toJsonString(depList);
+        }
     }
 
     public String getLoc() {