yandex_cdn_resource, absent custom_host_header field do not reset this field in CDN resource

[baznikin]

If you omit custom_host_header in resource definition any changes made with web console do not affect terraform apply - provider simply ignores it.

Steps to reproduce:

1. create resource

resource "yandex_cdn_resource" "test" {
  cname = "cdn-kb.example.com"
  origin_protocol = "https"
  origin_group_id = yandex_cdn_origin_group.cdn_kb.id
}

2. in web console change "main domain name" to custom Host with some value:
   
3. terraform apply show nothing changed from provider point of view:

No changes. Your infrastructure matches the configuration.

4. add custom_host_header to Terraform resource:

resource "yandex_cdn_resource" "test" {
  cname = "cdn-kb.example.com"
  origin_protocol = "https"
  origin_group_id = yandex_cdn_origin_group.cdn_kb.id
  options {
    custom_host_header = "aaa"
  }
}

5. now terraform apply plan this change:

  # yandex_cdn_resource.cdn_kb will be updated in-place
  ~ resource "yandex_cdn_resource" "test" {
        id              = "bc8tcqvnwrgo3bs6vug3"
        # (7 unchanged attributes hidden)

      ~ options {
          ~ custom_host_header         = "sss" -> "aaa"
            # (20 unchanged attributes hidden)
        }

        # (1 unchanged block hidden)
    }

This issue also persist with terraform import of resource with custom host header:

1. custom_host_header added to state
2. terraform apply do not plan to remove it if there is no custom_host_header option in resource

[opportunity356]

Issue is actual. Reproduces

[baznikin]

This is because of common provider (mis)behavior I mentioned in #465 - if there is no value at all in configuration it completly ignore it and do not try to sync