Fix region diff suppression for undefined regions

[dgadelha]

This is an attempt to fix region diff suppression when regions are not manually defined in code:

# betteruptime_monitor.test will be updated in-place
~ resource "betteruptime_monitor" "test" {
      id                  = "123456"
    ~ regions             = [
        - "us",
        - "eu",
        - "as",
        - "au",
      ]
      # (17 unchanged attributes hidden)
  }

With this resource:

resource "betteruptime_monitor" "test" {
  monitor_type = "status"
  url          = "https://google.com/"
}

As I'm not a Go/TF developer, please review this PR carefully.

[dgadelha]

Converting to draft due to hashicorp/terraform-plugin-sdk#477