[SPARK-28005][YARN] Remove unnecessary log from SparkRackResolver

## What changes were proposed in this pull request?

SparkRackResolver generates an INFO message every time is called with 0 arguments.
In this PR I've deleted it because it's too verbose.

## How was this patch tested?

Existing unit tests + spark-shell.

Closes apache#24935 from gaborgsomogyi/SPARK-28005.

Authored-by: Gabor Somogyi <[email protected]>
Signed-off-by: Imran Rashid <[email protected]>

Author: gaborgsomogyi

resource-managers/yarn/src/main/scala/org/apache/spark/deploy/yarn/SparkRackResolver.scala

@@ -67,6 +67,9 @@ private[spark] class SparkRackResolver(conf: Configuration) extends Logging {
   }
 
   private def coreResolve(hostNames: Seq[String]): Seq[Node] = {
+    if (hostNames.isEmpty) {
+      return Seq.empty
+    }
     val nodes = new ArrayBuffer[Node]
     // dnsToSwitchMapping is thread-safe
     val rNameList = dnsToSwitchMapping.resolve(hostNames.toList.asJava).asScala