fix a bug where rrdatas can not be unset in google_dns_record_set

[shuyama1]

Fixes b/234631896

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• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
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• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

dns: fixed a bug where `google_dns_record_set` resource can not be changed from default routing to Geo routing policy.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 35 insertions(+))
Terraform Beta: Diff ( 2 files changed, 35 insertions(+))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2034
Passed tests 1804
Skipped tests: 226
Failed tests: 4

Action taken

Triggering VCR tests in RECORDING mode for the tests that failed during VCR. Click here to see the failed tests

TestAccFirebaserulesRelease_BasicRelease|TestAccContainerCluster_withConfidentialNodes|TestAccContainerCluster_withAddons|TestAccDNSRecordSet_changeRouting

[modular-magician]

Tests passed during RECORDING mode:
TestAccFirebaserulesRelease_BasicRelease[view]
TestAccDNSRecordSet_changeRouting[view]

Tests failed during RECORDING mode:
TestAccContainerCluster_withAddons[view]
TestAccContainerCluster_withConfidentialNodes[view]

Please fix these to complete your PR
View the build log or the debug log for each test

[rileykarson]

What I can't figure out is how we were returning true in the first place-if old has any elements and new doesn't, shouldn't we have returned false during rrdatasListDiffSuppress?

[rileykarson]

(note you'll probably want a diff reviewer here since I'll be out!)

[shuyama1]

@rileykarson Sorry! Didn't see the comments in this PR somehow! I'll probably close this one as PR #6076 makes the similar changes, and we will work out the fix there.

Back to your question for the PR:
We use d.GetChange("rrdatas") in the rrdatasListDiffSuppress and it seems not detect the case when rrdatas is unset during update (rrdatas was previous set during creation).

[shuyama1]

Haven't heard back from #6076. Moving forward with PR then.