fix: set default empty list for `prefect_variable.tags`

[parkedwards]

resolves https://github.com/PrefectHQ/terraform-provider-prefect/issues/110

with this configuration:

resource "prefect_variable" "example" {
  name  = "my_variable"
  value = "variable value goes here"
  workspace_id = data.prefect_workspace.evergreen.id
}

this is the plan that is generated (and applies successfully)

adding new tags, modifying tags, and resetting the resource to be tag-less all apply correctly