🐛(frontend) draftBody can be null

There was an error in the openapi schema and we are not aware that
message.draftBody could be null so when it was the case, the frontend
throws an error when rendering the `MessageForm` component.

src/backend/core/api/openapi.json

@@ -4696,6 +4696,7 @@
                     },
                     "draftBody": {
                         "type": "string",
+                        "nullable": true,
                         "readOnly": true
                     },
                     "attachments": {