Cannot unset `acls_external_link` in `tailscale_tailnet_settings`

[zofrex]

Describe the bug
acls_external_link can be set, but not unset.

To Reproduce
Steps to reproduce the behaviour:

1. Set up the provider against a Tailnet that currently has no acls_external_link set
2. You can set the external link:

resource "tailscale_tailnet_settings" "test_settings" {
  acls_external_link = "https://example.com"
}

3. Now try to remove it. Setting the property to null, or removing it, has no effect. This is expected, not setting a setting is a no-op with this resource:

resource "tailscale_tailnet_settings" "test_settings" {
  # acls_external_link = "https://example.com"
}

tailscale_tailnet_settings.test_settings: Refreshing state... [id=singleton]

No changes. Your infrastructure matches the configuration.

4. But setting it to the empty string fails:

resource "tailscale_tailnet_settings" "test_settings" {
  acls_external_link = ""
}

│ Error: expected "acls_external_link" url to not be empty, got
│
│   with tailscale_tailnet_settings.test_settings,
│   on main.tf line 113, in resource "tailscale_tailnet_settings" "test_settings":
│  113:   acls_external_link = ""

Expected behaviour
There should be a way to unset acls_external_link, probably via passing the empty string.

Desktop (please complete the following information):

• OS: macOS
• Terraform Version: v1.5.5
• Provider Version: v0.28.0