[SPARK-27624][CORE] Fix CalenderInterval to show an empty interval correctly

Author: dongjoon-hyun

common/unsafe/src/main/java/org/apache/spark/unsafe/types/CalendarInterval.java

@@ -321,6 +321,10 @@ public String toString() {
       appendUnit(sb, rest, "microsecond");
     }
 
+    if (months == 0 && microseconds == 0) {
+      sb.append(" 0 microseconds");
+    }
+
     return sb.toString();
   }
 

common/unsafe/src/test/java/org/apache/spark/unsafe/types/CalendarIntervalSuite.java

@@ -41,6 +41,9 @@ public void equalsTest() {
   public void toStringTest() {
     CalendarInterval i;
 
+    i = new CalendarInterval(0, 0);
+    assertEquals("interval 0 microseconds", i.toString());
+
     i = new CalendarInterval(34, 0);
     assertEquals("interval 2 years 10 months", i.toString());