Nil return for absent Bigtable resources

[BrianPi]

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

Release Note Template for Downstream PRs (will be copied)

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[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

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[BrianPi]

@googlebot I signed it!

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[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 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#1597
depends: hashicorp/terraform-provider-google#5331

[emilymye]

@BrianPi - I'm going to close and recreate this PR (with your same commit) - our autogeneration handles PRs from a master branch poorly

[emilymye]

Recreated in #2935