return NULL at end of send_progress_thread

Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Jason Lee <jasonlee@lanl.gov>
Closes openzfs#16074

lib/libzfs/libzfs_sendrecv.c

@@ -1053,6 +1053,7 @@ send_progress_thread(void *arg)
 		}
 	}
 	pthread_cleanup_pop(B_TRUE);
+	return (NULL);
 }
 
 static boolean_t