MINOR: Record all poll invocations (#9234)

Record the pollSensor after every invocation to poll, rather than just when we get records back so that we can accurately gauge how often we're invoking Consumer#poll.

Reviewers: Bruno Cadonna <bruno@confluent.io>, Guozhang Wang <guozhang@apache.org>, Matthias J. Sax <mjsax@apache.org>

streams/src/main/java/org/apache/kafka/streams/processor/internals/StreamThread.java

@@ -636,8 +636,8 @@ public class StreamThread extends Thread {
 
         final long pollLatency = advanceNowAndComputeLatency();
 
+        pollSensor.record(pollLatency, now);
         if (records != null && !records.isEmpty()) {
-            pollSensor.record(pollLatency, now);
             pollRecordsSensor.record(records.count(), now);
             taskManager.addRecordsToTasks(records);
         }