test: prevent drift in netlify_site_domain_settings resource

Author: werwolfby

internal/provider/site_domain_settings_resource_test.go

@@ -54,3 +54,44 @@ func TestAccSiteDomainSettings(t *testing.T) {
 		},
 	}, func(s *terraform.State) error { return nil })
 }
+
+func TestAccSiteDomainSettingsNullFields(t *testing.T) {
+	// Test case for preventing drift when optional fields are null from API
+	accTest(t, []resource.TestStep{
+		{
+			Config: `resource "netlify_site_domain_settings" "null_fields" {
+  site_id       = "5b407d6d-9385-4e7a-a4c4-8efc11ea3c26"
+  custom_domain = "tf-test-null-fields.examplepetstore.com"
+}`,
+			Check: resource.ComposeAggregateTestCheckFunc(
+				resource.TestCheckResourceAttr("netlify_site_domain_settings.null_fields", "site_id", "5b407d6d-9385-4e7a-a4c4-8efc11ea3c26"),
+				resource.TestCheckResourceAttr("netlify_site_domain_settings.null_fields", "custom_domain", "tf-test-null-fields.examplepetstore.com"),
+				resource.TestCheckResourceAttr("netlify_site_domain_settings.null_fields", "domain_aliases.#", "0"),
+				// These should be null/empty when not set
+				resource.TestCheckNoResourceAttr("netlify_site_domain_settings.null_fields", "branch_deploy_custom_domain"),
+				resource.TestCheckNoResourceAttr("netlify_site_domain_settings.null_fields", "deploy_preview_custom_domain"),
+			),
+		},
+		{
+			// Second step with same config should not detect changes (no drift)
+			Config: `resource "netlify_site_domain_settings" "null_fields" {
+  site_id       = "5b407d6d-9385-4e7a-a4c4-8efc11ea3c26"
+  custom_domain = "tf-test-null-fields.examplepetstore.com"
+}`,
+			PlanOnly: true,
+			Check: resource.ComposeAggregateTestCheckFunc(
+				resource.TestCheckResourceAttr("netlify_site_domain_settings.null_fields", "site_id", "5b407d6d-9385-4e7a-a4c4-8efc11ea3c26"),
+				resource.TestCheckResourceAttr("netlify_site_domain_settings.null_fields", "custom_domain", "tf-test-null-fields.examplepetstore.com"),
+				resource.TestCheckResourceAttr("netlify_site_domain_settings.null_fields", "domain_aliases.#", "0"),
+			),
+		},
+		{
+			ResourceName:                         "netlify_site_domain_settings.null_fields",
+			ImportState:                          true,
+			ImportStateId:                        "5b407d6d-9385-4e7a-a4c4-8efc11ea3c26",
+			ImportStateVerifyIdentifierAttribute: "site_id",
+			ImportStateVerify:                    true,
+			ImportStateVerifyIgnore:              []string{"last_updated"},
+		},
+	}, func(s *terraform.State) error { return nil })
+}