Update db migration: allow nulls

DB must allow nulls otherwise this will break other monitors.

db/knex_migrations/2024-04-26-0000-snmp-monitor.js

@@ -2,9 +2,10 @@ exports.up = function (knex) {
     return knex.schema
         .alterTable("monitor", function (table) {
             table.string("snmp_community_string", 255).defaultTo("public"); // Add snmp_community_string column
-            table.string("snmp_oid").notNullable(); // Add oid column
+            table.string("snmp_oid").defaultTo(null); // Add oid column
             table.enum("snmp_version", ["1", "2c", "3"]).defaultTo("2c"); // Add snmp_version column with enum values
-            table.float("snmp_control_value").notNullable(); // Add control_value column as float
-            table.string("snmp_condition").notNullable(); // Add oid column
+            table.float("snmp_control_value").defaultTo(null); // Add control_value column as float
+            table.string("snmp_condition").defaultTo(null); // Add oid column
         });
+};
 };