Shutdown DLQ segments flusher only if it has been started (#15649)

In DLQ unit testing sometime the DLQ writer is started explicitly without starting the segments flushers. In such cases the test 's logs contains exceptions which could lead to think that the test fails silently.

Avoid to invoke scheduledFlusher's shutdown when it's not started (such behaviour is present only in tests).

logstash-core/src/main/java/org/logstash/common/io/DeadLetterQueueWriter.java

@@ -254,7 +254,10 @@ public void close() {
             }
 
             try {
-                flushScheduler.shutdown();
+                // flushScheduler is null only if it's not explicitly started, which happens only in tests.
+                if (flushScheduler != null) {
+                    flushScheduler.shutdown();
+                }
             } catch (Exception e) {
                 logger.warn("Unable shutdown flush scheduler, ignoring", e);
             }