fix apache#10781 Build failure because of spotbugs (apache#10792)

Fixes apache#10781 

### Motivation

solving build failure case causing by spotbugs issue which is explained in apache#10781 

### Modifications

Small modification: add a null check in BKStateStoreImpl.java:[line 175]

pulsar-functions/instance/src/main/java/org/apache/pulsar/functions/instance/state/BKStateStoreImpl.java

@@ -172,7 +172,9 @@ public CompletableFuture<ByteBuffer> getAsync(String key) {
                         }
                         return null;
                     } finally {
-                        ReferenceCountUtil.safeRelease(data);
+                        if (data != null) {
+                            ReferenceCountUtil.safeRelease(data);
+                        }
                     }
                 }
         );