Fix config generation error for zmq-enabled on upgrade

The field was moved to the top level but not marked nullable, so
upgrades from older configs (where it lived under advanced) failed
with "Field Is Not Nullable". The entrypoint already falls back to
true via yq's // operator.

scripts/procedures/getConfig.ts

@@ -109,6 +109,7 @@ export const getConfig: T.ExpectedExports.getConfig = compat.getConfig({
     "name": "Enable ZMQ Block Notifications",
     "description":
       "Subscribe to Bitcoin Core's hashblock ZMQ topic for sub-second block detection on the dashboard. Requires zmqpubhashblock enabled on bitcoind.",
+    "nullable": true,
     "default": true,
   },
   "advanced": {