[SPARK-27243][SQL] RuleExecutor.dumpTimeSpent should not throw exception when empty

## What changes were proposed in this pull request?

`RuleExecutor.dumpTimeSpent` currently throws an exception when invoked before any rule is run or immediately after `RuleExecutor.reset`. The PR makes it returning an empty summary, which is the expected output instead.

## How was this patch tested?

added UT

Closes apache#24180 from mgaido91/SPARK-27243.

Authored-by: Marco Gaido <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: mgaido91

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/rules/QueryExecutionMetering.scala

@@ -64,7 +64,11 @@ case class QueryExecutionMetering() {
   /** Dump statistics about time spent running specific rules. */
   def dumpTimeSpent(): String = {
     val map = timeMap.asMap().asScala
-    val maxLengthRuleNames = map.keys.map(_.toString.length).max
+    val maxLengthRuleNames = if (map.isEmpty) {
+      0
+    } else {
+      map.keys.map(_.toString.length).max
+    }
 
     val colRuleName = "Rule".padTo(maxLengthRuleNames, " ").mkString
     val colRunTime = "Effective Time / Total Time".padTo(len = 47, " ").mkString

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/trees/RuleExecutorSuite.scala

@@ -91,4 +91,10 @@ class RuleExecutorSuite extends SparkFunSuite {
     }.getMessage
     assert(message.contains("the structural integrity of the plan is broken"))
   }
+
+  test("SPARK-27243: dumpTimeSpent when no rule has run") {
+    RuleExecutor.resetMetrics()
+    // This should not throw an exception
+    RuleExecutor.dumpTimeSpent()
+  }
 }