netlify_site_domain_settings_changes fix update in place without changes

[werwolfby]

This error fixes state when I specify netlify_site_domain_settings_changes resource like this:

resource "netlify_site_domain_settings" "my_site" {
  site_id                      = data.netlify_site.my_site.id
  custom_domain       = "www.my.site"
}

This cause changes on each plan/apply:

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  # netlify_site_domain_settings.my_site will be updated in-place
  ~ resource "netlify_site_domain_settings" "my_site" {
      ~ last_updated                 = "Wednesday, 25-Jun-25 22:58:25 PDT" -> (known after apply)
        # (5 unchanged attributes hidden)
    }

This changes fixes the issue.

This pull request improves the handling of optional fields in the site_domain_settings resource by ensuring null values are explicitly managed and adds a new test case to prevent configuration drift. The changes enhance both the functionality and reliability of the resource.

Enhancements to site_domain_settings resource:

• Updated the Read method in internal/provider/site_domain_settings_resource.go to explicitly set optional fields (CustomDomain, BranchDeployCustomDomain, and DeployPreviewCustomDomain) to StringNull when they are not provided by the API. This ensures consistent state representation for optional fields.

New test case for preventing configuration drift:

• Added a new test function TestAccSiteDomainSettingsNullFields in internal/provider/site_domain_settings_resource_test.go. This test verifies that optional fields remain null when not set and ensures no drift occurs when reapplying the same configuration. It also includes an import verification step.