stat: fix wrong check for whether we have a description or not

It's an array, hence never NULL. Check for size instead.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/stat.c b/stat.c
index 5a553d0..ca73310 100644
--- a/stat.c
+++ b/stat.c
@@ -831,7 +831,7 @@
 	log_info("\n");
 
 	/* Additional output if description is set */
-	if (ts->description)
+	if (strlen(ts->description))
 		log_info(";%s", ts->description);
 
 	log_info("\n");