data block with for_each evaluating to empty collection does not register namespace, breaking downstream references

[jaredfholgate]

Summary

When a data block has for_each that evaluates to an empty collection, the resulting namespace (data.<type>.<name>) is never registered in the evaluation context. Any downstream reference (e.g. for_each = data.<type>.<name> on another data block or a transform) throws an "unknown reference" error instead of degrading to an empty iteration.

This forces every consumer of a for_each-driven data namespace to wrap their reference in try(data.X.Y, {}), which is ergonomically poor and easy to forget.

Reproducer

# .mptf.hcl
data "module" "for_order" {}

# When the tf-dir has no `module "..." {}` calls, this evaluates 0 times.
data "module_source" "for_order" {
  for_each = data.module.for_order.result
  source   = each.value.source
}

transform "reorder_attributes" "module_full" {
  for_each             = data.module_source.for_order   # ❌ data.module_source.for_order unknown
  target_block_address = each.value.module_address
  ...
}

Running mapotf transform against a tf-dir with zero modules errors with unknown reference: data.module_source.for_order.

Current workaround

Wrap the consumer with try():

transform "reorder_attributes" "module_full" {
  for_each = try(data.module_source.for_order, {})
  ...
}

The Azure AVM governance pipeline ships this workaround in mapotf-configs/order_module_attrs.mptf.hcl at https://github.com/Azure/avm-terraform-governance/blob/main/mapotf-configs/order_module_attrs.mptf.hcl with a comment block explaining the quirk. Cost is roughly nil at runtime but it's a foot-gun for anyone authoring new for_each-driven chains.

Suggested fix

In the data-block plan loop, when for_each is non-null but the evaluated collection is empty:

1. Still register the data.<type>.<name> namespace in the eval context.
2. Bind it to an empty cty.ObjectVal{} (or equivalent empty collection matching the block's value type).

This mirrors Terraform's own behaviour where for_each = [] on a resource produces an empty map under <resource_type>.<name> rather than an unknown reference.

Estimated cost: ~5 lines in the data-block iteration path, plus one test.

Impact

• Not a regression — pre-existing behaviour back to v0.1.0
• Low severity (documented try() workaround is reliable)
• High ergonomic win (eliminates a documented foot-gun for data.module / data.module_source chains)

Credit

Surfaced during real-world deployment in the Azure AVM Terraform governance pipeline alongside the v0.1.4 cleanup PR (Azure/avm-terraform-governance#472). Reported by the governance pipeline maintainer; verified in mapotf v0.1.4.