chore(coderd/database): enforce workspace ACL JSON object constraints (#22019)

The constraints prevent faulty code from saving 'null' as JSON and breaking the `workspaces_expanded` view.

coderd/database/migrations/testdata/fixtures/000416_pre_workspace_acl_object_constraint.up.sql

@@ -0,0 +1,35 @@
+-- Fixture for migration 000417_workspace_acl_object_constraint.
+-- Inserts a workspace with 'null'::json ACLs to ensure the migration
+-- correctly normalizes such values.
+
+INSERT INTO workspaces (
+    id,
+    created_at,
+    updated_at,
+    owner_id,
+    organization_id,
+    template_id,
+    deleted,
+    name,
+    last_used_at,
+    automatic_updates,
+    favorite,
+    group_acl,
+    user_acl
+)
+VALUES (
+    '6f6fdbee-4c18-4a5c-8a8d-9b811c9f0a28',
+    '2024-02-10 00:00:00+00',
+    '2024-02-10 00:00:00+00',
+    '30095c71-380b-457a-8995-97b8ee6e5307',
+    'bb640d07-ca8a-4869-b6bc-ae61ebb2fda1',
+    '4cc1f466-f326-477e-8762-9d0c6781fc56',
+    false,
+    'acl-null-workspace',
+    '0001-01-01 00:00:00+00',
+    'never',
+    false,
+    'null'::jsonb,
+    'null'::jsonb
+)
+ON CONFLICT DO NOTHING;