Use == operator for shared_ptr nullptr comparison (#9465)

Summary:
From C++ 20 onwards, the != operator is not supported for a shared_ptr.
So switch to using ==.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9465

Test Plan: make check

Reviewed By: riversand963

Differential Revision: D33850596

Pulled By: anand1976

fbshipit-source-id: eec16d1aa6c39a315ec2d44d233d7518f9c1ddcb

include/rocksdb/utilities/object_registry.h

@@ -531,10 +531,10 @@ class ObjectRegistry {
         }
       }
     }
-    if (parent_ != nullptr) {
-      return parent_->FindFactory<T>(name);
-    } else {
+    if (parent_ == nullptr) {
       return nullptr;
+    } else {
+      return parent_->FindFactory<T>(name);
     }
   }