* Set a register value to persist in state, even after
the value is no longer set in the Terraform resource
* Setting the value to `null` or "" (empty string)
won't alter the resource value (or show a diff)
* Allow the value to be updated (plan shows a diff)
This differs from conditional expressions that check that
a value isn't null or default to a value (which may be a
data reference to the old value):
```tf
locals {
out = var.foo == null ? data.old_foo : var.foo
}
data "old_foo" ...
```
Instead, it allows a Terraform resource value to retain
a value from a previous declarative state. This can be
useful in cases where querying and referencing the value
again is expensive and the value cannot change externally.
```tf
resource "util_register" "example" {
set = null # previously "bar"
}
output "out" {
value = util_register.example.value # "bar"
}
```
