Allow deserializing empty compound tags in deserialize_any

[Iaiao]

Now it returns an error in deserialize_map if the tag is not 0x0A instead of just returning an error in deserialize_any.

let empty = nbt::Value::Compound(HashMap::new());
let mut serialized = Vec::new();
nbt::to_writer(&mut serialized, &empty, None).unwrap();
assert_eq!(serialized, vec![0x0A, 0x00, 0x00, 0x00]);
let should_work: nbt::Value = nbt::from_reader(Cursor::new(serialized)).unwrap(); // Error::NoRootCompound