Can't deserialize Arrow Schema with empty metadata?

[Jack Chan]

Hi

I tried to serialize and deserialize an arrow schema, but I got into an
error of
(Error { message: "missing field `metadata`" }). Is this a bug in apache
arrow or in serde_dynamodb?

```
        let schema = ArrowSchema::empty();
        let object = serde_dynamodb::to_hashmap(&schema)?;
        let roundtrip: ArrowSchema = serde_dynamodb::from_hashmap(object)?;
        assert_eq!(roundtrip, schema);
```

Jack