fix: Uses google_cloud_kms_config correctly in mongodbatlas_encryption_at_rest creation

[DasLampe]

Description

Fix mongodbatlas_encryption_at_rest terraform resource, when google_cloud_kms_config block is defined.
Because of the wrong check-in if statement the request body to MongoDB Atlas was empty if there was no azure_key_vault_config block, which resulted in a MISSING_ENCRYPTION_AT_REST_PROVIDER error.

Type of change:

• [ x] Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR.
• This change requires a documentation update
• Documentation fix/enhancement

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• [x ] I have signed the MongoDB CLA
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• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
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Further comments

[AgustinBettati]

LGTM, thank you for the contribution!

[AgustinBettati]

Was able to reproduce the error and verified the fix when defining google_cloud_kms_config.
Ran the TestAccEncryptionAtRest_basicGCP locally (this test was catching the existing bug but is not run in our CI):

go test -timeout 2400s -run ^TestAccEncryptionAtRest_basicGCP$ github.com/mongodb/terraform-provider-mongodbatlas/internal/service/encryptionatrest

ok  	github.com/mongodb/terraform-provider-mongodbatlas/internal/service/encryptionatrest	5.601s

We can ignore the failing CI (due to PR coming from forked repository).