message: Implement Default for empty
diff: @@ -7,7 +7,7 @@ use serde::{Deserialize, Serialize};
  /// It is designed to be expressable in correct JSON and JSON Schema but
  /// contains no meaningful data. Previously we used enums without cases,
  /// but those cannot represented as valid JSON Schema (https://github.com/CosmWasm/cosmwasm/issues/451)
  -#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, JsonSchema)]
  +#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, JsonSchema, Default)]
  pub struct Empty {}
  #[cfg(test)]
  @@ -16,6 +16,12 @@ mod tests {
  use crate::serde::{from_slice, to_vec};
  + #[test]
  + fn empty_can_be_instantiated() {
  + let instance = Empty::default();
  + assert_eq!(instance, Empty {});
  + }
  +
  #[test]
  fn empty_can_be_instantiated_serialized_and_deserialized() {
  let instance = Empty {};
  