Merge pull request #3732 from actiontech/2232

fix npe  inner 2232

Author: ylinzhu

src/main/java/com/actiontech/dble/services/BackendService.java

@@ -335,10 +335,8 @@ public void backendSpecialCleanUp() {
 
     public void releaseSignal() {
         isRowDataFlowing = false;
-        if (recycler != null) {
-            recycler.signal();
-            recycler = null;
-        }
+        Optional.ofNullable(recycler).ifPresent(res -> res.signal());
+        recycler = null;
     }
 
     public void resetContextStatus() {