## 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 <dhyun@apple.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
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