make send_alerts_to field nullable (#45572)

* make send_alerts_to field nullable

* set nullable to true since we have a default value

pkg/services/sqlstore/migrations/ualert/tables.go

@@ -327,7 +327,7 @@ func AddAlertAdminConfigMigrations(mg *migrator.Migrator) {
 	mg.AddMigration("create_ngalert_configuration_table", migrator.NewAddTableMigration(adminConfiguration))
 	mg.AddMigration("add index in ngalert_configuration on org_id column", migrator.NewAddIndexMigration(adminConfiguration, adminConfiguration.Indices[0]))
 	mg.AddMigration("add column send_alerts_to in ngalert_configuration", migrator.NewAddColumnMigration(adminConfiguration, &migrator.Column{
-		Name: "send_alerts_to", Type: migrator.DB_SmallInt, Nullable: false,
+		Name: "send_alerts_to", Type: migrator.DB_SmallInt, Nullable: false, Default: "0",
 	}))
 }