Nil return for absent Bigtable resources

[emilymye]

Return nil after printing "[WARN] Removing %s because it's gone" instead
of an error for Bigtable resources, bringing behaviour in line with that
of other resource types.

Release Note Template for Downstream PRs (will be copied)

bigtable: Fixed error on reading non-existent `google_bigtable_gc_policy`,  `google_bigtable_instance`,  `google_bigtable_table`

[googlebot]

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[emilymye]

@BrianPi i'm going to override CLA since it was approved in #2927

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[nat-henderson]

/gcbrun

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
I see that this PR has already had some downstream PRs generated. Any open downstreams are already updated to your most recent commit, 8b593c3.

Pull request statuses

No diff detected in terraform-google-conversion.
No diff detected in terraform-provider-google.
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#1598

[googlebot]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

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