clear the data on init to make sure secret isn't returned

It doesn't currently actually return the migration secret in the data 
but it technically should, and will after this issue is fixed: scrtlabs/SecretNetwork#1323 (comment)

contracts/purchaseable-snip721-v1/src/contract.rs

@@ -87,10 +87,10 @@ pub(crate) fn init_snip721(
     let snip721_response = snip721_reference_impl::contract::instantiate(deps, env, info.clone(), instantiate_msg)
         .unwrap();
 
-    // clear the data (that contains the secret) which would be set when init_snip721 is called
+    // clear the data (that contains the secret) which would be set before init_snip721 is called
     // from reply as part of the migration process
     // https://github.com/CosmWasm/cosmwasm/blob/main/SEMANTICS.md#handling-the-reply
-    return Ok(snip721_response);
+    return Ok(snip721_response.set_data(b""));
 }