fix(cdn): error when optional blocked_countries field is not defined as empty list

[pavlov-tony]

Description

Customer reported an error on distribution creation with non specified optional blocked_countries property.

relates to STACKITCDN-1014

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[rubenhoenle]

Why are these changes here needed? Is it needed for the bugfix or just some refactoring?

In case it's just some refactoring and not explicitly needed, I would ask you to please split it into another PR.

And one note about acc tests: We try to use the following schema for our acc tests with embedding some *.tf files. So the Terraform config is not part of the Golang test code. I would like to ask you to stick to that format to keep everything consistent across the codebase. 😅

As you can see usually we have one Min test with a minimal config and one Max test for the full-blown config for each resource.

terraform-provider-stackit/stackit/internal/services/git/git_acc_test.go

Lines 22 to 26 in 6f4cb49

	//go:embed testdata/resource-min.tf
	var resourceMin string
	//go:embed testdata/resource-max.tf
	var resourceMax string

terraform-provider-stackit/stackit/internal/services/git/testdata/resource-min.tf

Lines 1 to 8 in 6f4cb49

	variable "project_id" {}
	variable "name" {}
	resource "stackit_git" "git" {
	project_id = var.project_id
	name = var.name
	}