fix: set default empty list for prefect_variable.tags

[parkedwards]

resolves #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

[parkedwards]

since this is a list type, there are these special constructors you need to use to set default values here

https://developer.hashicorp.com/terraform/plugin/framework/handling-data/attributes/list#plan-modification