OpenApi specification: strucural error on null types

[hideintheclouds]

According to the OpenApi specification 3.0.0, the null type is not supported.
Instead, the nullable attribute should be used: see:
https://swagger.io/docs/specification/data-models/data-types/

This issue is present in the OpenApi specification when defining a 200 successful response:
"responses": { "200": { "description": "", "content": { "application/json": { "schema": { "type": "null" } } } },

The following operations are impacted:

• /swap/deregister-utxo POST (https://github.com/commerceblock/mercury/blob/49ed815110bc7030470585f578ec16377dd8a993/layer/openapi.json#L466C28-L466C32)

• /swap/first POST

• /swap/register-utxo POST

• /transfer/batch/init POST

• /transfer/batch/reveal POST

• /transfer/keyupdate_complete POST

• /transfer/update_msg POST

• /withdraw/confirm POST