Fix a minor issue in checkpoint test case (#8483)

Summary:
A very simple change :)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8483

Reviewed By: ajkr

Differential Revision: D29558904

Pulled By: mrambacher

fbshipit-source-id: bbe68c20c861103726cb6231ca3fb8fbe1e5a546

utilities/checkpoint/checkpoint_test.cc

@@ -751,7 +751,8 @@ TEST_F(CheckpointTest, CheckpointInvalidDirectoryName) {
   for (std::string checkpoint_dir : {"", "/", "////"}) {
     Checkpoint* checkpoint;
     ASSERT_OK(Checkpoint::Create(db_, &checkpoint));
-    ASSERT_TRUE(checkpoint->CreateCheckpoint("").IsInvalidArgument());
+    ASSERT_TRUE(
+        checkpoint->CreateCheckpoint(checkpoint_dir).IsInvalidArgument());
     delete checkpoint;
   }
 }