[BUG][API]Fix format2Duration when parameter has one is null (#4802)

* fix format2Duration when parameter has one is null

dolphinscheduler-common/src/main/java/org/apache/dolphinscheduler/common/utils/DateUtils.java

@@ -259,6 +259,9 @@ public class DateUtils {
      * @return format time
      */
     public static String format2Duration(Date d1, Date d2) {
+        if (d1 == null || d2 == null) {
+            return null;
+        }
         return format2Duration(differMs(d1, d2));
     }
 

dolphinscheduler-common/src/test/java/org/apache/dolphinscheduler/common/utils/DateUtilsTest.java

@@ -196,4 +196,12 @@ public class DateUtilsTest {
 
     }
 
+    @Test
+    public void testNullDuration() {
+        // days hours minutes seconds
+        Date d1 = DateUtils.stringToDate("2020-01-20 11:00:00");
+        Date d2 = null;
+        Assert.assertNull(DateUtils.format2Duration(d1, d2));
+    }
+
 }