A plan cannot turn a configured [] into null

Trigger: An Optional attribute that is not computed has x = [] in the configuration, and a plan modifier sets the plan to null.

Proof: terraform plan fails with Provider produced invalid plan ... planned value cty.NullVal(...) does not match config value cty.ListValEmpty(...). For a nested list the text is planned for absence but config wants existence. It holds at the top level, in a list item, and for a ListNestedAttribute.

Why: Core accepts a plan that differs from a non-null configured value only when the plan equals a non-null prior value. A null plan for [] never qualifies, so the modifier cannot take effect.

Fix: Keep [] in the plan, and make the read give the plan back. readEmptyAs: "null" makes a response without values null, and generated state keeps the form ([] or null) that the plan or the prior state has. Do not add a plan-side key.

Check: A throwaway resource.UnitTest with a one-attribute provider shows it in seconds. Delete it afterwards.