[BUILD][MINOR] Fix 2.10 build

## What changes were proposed in this pull request?

https://github.com/apache/spark/pull/17385 breaks the 2.10 sbt/maven builds by hitting an empty-string interpolation bug (https://issues.scala-lang.org/browse/SI-7919).

https://amplab.cs.berkeley.edu/jenkins/view/Spark%20QA%20Compile/job/spark-master-compile-sbt-scala-2.10/4072/
https://amplab.cs.berkeley.edu/jenkins/view/Spark%20QA%20Compile/job/spark-master-compile-maven-scala-2.10/3987/

## How was this patch tested?

Compiles

Author: Sameer Agarwal <sameerag@cs.berkeley.edu>

Closes #17391 from sameeragarwal/build-fix.

sql/core/src/test/scala/org/apache/spark/sql/streaming/FlatMapGroupsWithStateSuite.scala

@@ -240,7 +240,7 @@ class FlatMapGroupsWithStateSuite extends StateStoreMetricsTest with BeforeAndAf
   // Tests for StateStoreUpdater.updateStateForKeysWithData() when timeout != NoTimeout
   for (priorState <- Seq(None, Some(0))) {
     for (priorTimeoutTimestamp <- Seq(NO_TIMESTAMP, 1000)) {
-      var testName = s""
+      var testName = ""
       if (priorState.nonEmpty) {
         testName += "prior state set, "
         if (priorTimeoutTimestamp == 1000) {