fix: integration should be unique per environment

The unique constraint for integration is for unique_key, environment_id
and deleted_at which can be null. However null values are not considered
equal by default by postgres. :/

from postgres doc at https://www.postgresql.org/docs/current/ddl-constraints.html#DDL-CONSTRAINTS-UNIQUE-CONSTRAINTS
> By default, two null values are not considered equal in this
comparison. That means even in the presence of a unique constraint it is
possible to store duplicate rows that contain a null value in at least
one of the constrained columns. This behavior can be changed by adding
the clause NULLS NOT DISTINCT

packages/shared/lib/db/migrations/20240320184257_integration-unique-constraint-nulls-not-distinct.cjs

@@ -0,0 +1,11 @@
+exports.up = async function (knex) {
+    return knex
+        .raw('ALTER TABLE _nango_configs DROP CONSTRAINT IF EXISTS _nango_configs_unique_key_environment_id_deleted_at_unique')
+        .then(() =>
+            knex.raw(
+                'ALTER TABLE _nango_configs ADD CONSTRAINT _nango_configs_unique_key_environment_id_deleted_at_unique UNIQUE NULLS NOT DISTINCT (unique_key, environment_id, deleted_at)'
+            )
+        );
+};
+
+exports.down = async function () {};