fix crash in serviceAccountDiffSuppress

[megan07]

Fixes hashicorp/terraform-provider-google#9022

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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).
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Release Note Template for Downstream PRs (will be copied)

compute: fixed bug where terraform would crash if updating from no `service_account.scopes` to more.

[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, 75 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 2 files changed, 75 insertions(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=184908"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccComputeBackendService_internalLoadBalancing|TestAccComputeInstance_serviceAccount_updated0to1to0scopes You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=184918"