message: making default simpler
diff: @@ -18,7 +18,7 @@ pub struct Proof {
  pub struct RequestedProof {
  pub revealed_attrs: HashMap<String, RevealedAttributeInfo>,
  #[serde(skip_serializing_if="HashMap::is_empty")]
  - #[serde(default = "HashMap::new")]
  + #[serde(default)]
  pub revealed_attr_groups: HashMap<String, RevealedAttributeGroupInfo>,
  pub self_attested_attrs: HashMap<String, String>,
  pub unrevealed_attrs: HashMap<String, SubProofReferent>,
  