Update CT resource batch_id plan modifiers to handle unknown children

[chrismarget-j]

It turns out that #1017 fixed a related issue, but not the exact problem faced by @coterv in #1012.

Both issues are in the plan modifier for the CT primitive batch_id attribute.

@coterv's configuration causes the plan modifier to run while the details of child primitives are in Unknown state.

Prior to these changes, the plan modifier counts those not-yet-available child primitives, concludes that none exist, and sets the batch_id attribute to null (there are no children, so there won't be a child container, so the ID is moot).

Later, when the child primitives are fully fleshed out (upstream dependencies are satisfied), the plan modifier runs again. This time, when it counts the child primitives, it finds a non-zero number and sets the batch_id to Unknown (there will be a child container object, and it will have an ID).

Terraform complains about this inconsistent planning behavior: batch_id: was null, but now cty.UnknownVal(cty.String)

Closes #1012

[coterv]

Issue resolved in v0.80.1