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

## What changes were proposed in this pull request?

If the interval is `0`, it doesn't show both the value `0` and the unit at all. For example, this happens in the explain plans and Spark Web UI on `EventTimeWatermark` diagram.

**BEFORE**
```scala
scala> spark.readStream.schema("ts timestamp").parquet("/tmp/t").withWatermark("ts", "1 microsecond").explain
== Physical Plan ==
EventTimeWatermark ts#0: timestamp, interval 1 microseconds
+- StreamingRelation FileSource[/tmp/t], [ts#0]

scala> spark.readStream.schema("ts timestamp").parquet("/tmp/t").withWatermark("ts", "0 microsecond").explain
== Physical Plan ==
EventTimeWatermark ts#3: timestamp, interval
+- StreamingRelation FileSource[/tmp/t], [ts#3]
```

**AFTER**
```scala
scala> spark.readStream.schema("ts timestamp").parquet("/tmp/t").withWatermark("ts", "1 microsecond").explain
== Physical Plan ==
EventTimeWatermark ts#0: timestamp, interval 1 microseconds
+- StreamingRelation FileSource[/tmp/t], [ts#0]

scala> spark.readStream.schema("ts timestamp").parquet("/tmp/t").withWatermark("ts", "0 microsecond").explain
== Physical Plan ==
EventTimeWatermark ts#3: timestamp, interval 0 microseconds
+- StreamingRelation FileSource[/tmp/t], [ts#3]
```

## How was this patch tested?

Pass the Jenkins with the updated test case.

Closes #24516 from dongjoon-hyun/SPARK-27624.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>
(cherry picked from commit 614a5cc)
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

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

@@ -319,6 +319,8 @@ public String toString() {
       appendUnit(sb, rest / MICROS_PER_MILLI, "millisecond");
       rest %= MICROS_PER_MILLI;
       appendUnit(sb, rest, "microsecond");
+    } else if (months == 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());