[SPARK-23245][SS][TESTS] Don't access lastExecution.executedPlan in StreamTest

## What changes were proposed in this pull request?

`lastExecution.executedPlan` is lazy val so accessing it in StreamTest may need to acquire the lock of `lastExecution`. It may be waiting forever when the streaming thread is holding it and running a continuous Spark job.

This PR changes to check if `s.lastExecution` is null to avoid accessing `lastExecution.executedPlan`.

## How was this patch tested?

Jenkins

Author: Jose Torres <jose@databricks.com>

Closes #20413 from zsxwing/SPARK-23245.

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

@@ -472,7 +472,7 @@ trait StreamTest extends QueryTest with SharedSQLContext with TimeLimits with Be
               currentStream.awaitInitialization(streamingTimeout.toMillis)
               currentStream match {
                 case s: ContinuousExecution => eventually("IncrementalExecution was not created") {
-                  s.lastExecution.executedPlan // will fail if lastExecution is null
+                  assert(s.lastExecution != null)
                 }
                 case _ =>
               }