[Improve] The start time may be null.#4140 (#4141)

Author: Forus0322

streampark-console/streampark-console-service/src/main/java/org/apache/streampark/console/core/bean/AlertTemplate.java

@@ -79,7 +79,7 @@ public AlertTemplateBuilder endTime(Date endTime) {
 
         public AlertTemplateBuilder duration(Date start, Date end) {
             long duration;
-            if (start == null && end == null) {
+            if (start == null) {
                 duration = 0L;
             } else if (end == null) {
                 duration = System.currentTimeMillis() - start.getTime();