Don't make ACL comparisons if objects are missing

[chrisst]

Fixes hashicorp/terraform-provider-google#3949

Release Note for Downstream PRs (will be copied)

`google_storage_object_acl` Prevent panic when using interpolated object names.

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
This PR seems not to have generated downstream PRs before, as of 42754f9.

Pull request statuses

No diff detected in terraform-google-conversion.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I built this PR into one or more new PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: hashicorp/terraform-provider-google-beta#917
depends: hashicorp/terraform-provider-google#3970

[rileykarson]

Would it be worth logging the reason if we exit early when either field is missing? That way we can review exactly what's happening in debug logs a little more easily.

[chrisst]

When customize diff is running on plan and the resources that are meant to be interpolated are missing it's always going to exit at these points. Also both of these fields are required so the only reason this will exit early is during pre-creation plans.
Given that this is pretty normal exit condition I don't think the added log really adds anything.