Fix tests due to null stream transport passed to StubbableTransport

Signed-off-by: Rishabh Maurya <rishabhmaurya05@gmail.com>

Author: rishabhmaurya

test/framework/src/main/java/org/opensearch/test/transport/MockTransportService.java

@@ -265,7 +265,7 @@ public MockTransportService(
         this(
             settings,
             new StubbableTransport(transport),
-            new StubbableTransport(streamTransport),
+            streamTransport != null ? new StubbableTransport(streamTransport) : null,
             threadPool,
             interceptor,
             localNodeFactory,