Send empty log_config value

[c2thorn]

Fixes hashicorp/terraform-provider-google#9943

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Release Note Template for Downstream PRs (will be copied)

compute: fixed an issue in `google_compute_router_nat` where removing `log_config` resulted in a perma-diff

[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, 95 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 2 files changed, 95 insertions(+), 2 deletions(-))

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccComputeRouterNat_removeLogConfig|TestAccComputeRouterNat_withManualIpAndSubnetConfiguration|TestAccComputeRouterNat_withDisabledIndependentEndpointMapping|TestAccComputeRouterNat_withNatIpsAndDrainNatIps|TestAccPrivatecaCaPool_privatecaCapoolEmptyBaseline You can view the result here: https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=203500

[modular-magician]

Tests failed during RECORDING mode: TestAccPrivatecaCaPool_privatecaCapoolEmptyBaseline Please fix these to complete your PR