Deleting `preview_tmpl` does not actually empty the field on apply

[demeyerthom]

Example:

resource "storyblok_component" "three_columns" {
  name                 = "three_columns"
  space_id             = var.variables.storyblok_space_id
  is_root              = false
  is_nestable          = true
  display_name         = "3 Columns"
  component_group_uuid = storyblok_component_group.content.uuid

  schema = {
    column_1 = {
      position     = 0
      display_name = "Column 1"
      required     = true
      type         = "bloks"
      component_whitelist = [
        "buttons",
        "rich_text"
      ]
      restrict_components = true
    }
    column_2 = {
      position     = 1
      display_name = "Column 2"
      required     = true
      type         = "bloks"
      component_whitelist = [
        "image",
        "buttons",
        "rich_text"
      ]
      restrict_components = true
    }
    column_3 = {
      position     = 2
      display_name = "Column 3"
      required     = true
      type         = "bloks"
      component_whitelist = [
        "image",
        "buttons",
        "rich_text"
      ]
      restrict_components = true
    }
  }
  ==> preview_tmpl = "delete-me"
}

Removing preview_tmpl causes the following error:

Field is not emptied: