Overwrite RETL model resource schedule values even if null

[deanhuynh]

Since these fields are deprecated and no longer passed to Public API, we are overwriting them with whatever the previous value was to prevent recurring diffs. This has no impact on actual applies since this field is ignored, but it was confusing to see the diff continually pop up.

This prevents changes like this from recurring:

Terraform will perform the following actions:

  # segment_reverse_etl_model.example will be updated in-place
  ~ resource "segment_reverse_etl_model" "example" {
        id                      = "abc123"
        name                    = "name"
      - schedule_config         = jsonencode({})
        # (5 unchanged attributes hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.

Addresses #160