[MINOR][CORE] Fix a info message of `prunePartitions`

## What changes were proposed in this pull request?

`PrunedInMemoryFileIndex.prunePartitions` shows `pruned NaN% partitions` for the following case.

```scala
scala> Seq.empty[(String, String)].toDF("a", "p").write.partitionBy("p").saveAsTable("t1")

scala> sc.setLogLevel("INFO")

scala> spark.table("t1").filter($"p" === "1").select($"a").show
...
17/03/13 00:33:04 INFO PrunedInMemoryFileIndex: Selected 0 partitions out of 0, pruned NaN% partitions.
```

After this PR, the message looks like this.
```scala
17/03/15 10:39:48 INFO PrunedInMemoryFileIndex: Selected 0 partitions out of 0, pruned 0 partitions.
```

## How was this patch tested?

Pass the Jenkins with the existing tests.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #17273 from dongjoon-hyun/SPARK-EMPTY-PARTITION.

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/PartitioningAwareFileIndex.scala

@@ -186,7 +186,8 @@ abstract class PartitioningAwareFileIndex(
         val total = partitions.length
         val selectedSize = selected.length
         val percentPruned = (1 - selectedSize.toDouble / total.toDouble) * 100
-        s"Selected $selectedSize partitions out of $total, pruned $percentPruned% partitions."
+        s"Selected $selectedSize partitions out of $total, " +
+          s"pruned ${if (total == 0) "0" else s"$percentPruned%"} partitions."
       }
 
       selected