fix not setting a default for json monitor operations (louislam#5295)

db/knex_migrations/2024-10-31-0000-fix-snmp-monitor.js

@@ -0,0 +1,7 @@
+exports.up = function (knex) {
+    return knex("monitor").whereNull("json_path_operator").update("json_path_operator", "==");
+};
+exports.down = function (knex) {
+    // changing the json_path_operator back to null for all "==" is not possible anymore
+    // we have lost the context which fields have been set explicitely in >= v2.0 and which would need to be reverted
+};