length check added before type assertion to avoid panic

[venkykuberan]

If this PR is for Terraform, I acknowledge that I have:

• 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.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• 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)

storage - added length check for `cors` for the resource `resource_storage_bucket`

fixes hashicorp/terraform-provider-google#7690

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 1 file changed, 3 insertions(+))
Terraform Beta: Diff ( 1 file changed, 3 insertions(+))

[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=155618"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccDataSourceGoogleServiceAccountAccessToken_basic|TestAccDataSourceIAMBetaWorkloadIdentityPool_basic|TestAccDataSourceSpannerInstance_basic|TestAccBigqueryDataTransferConfig|TestAccResourceGoogleProjectDefaultServiceAccountsBasic|TestAccResourceGoogleProjectDefaultServiceAccountsDisable|TestAccResourceGoogleProjectDefaultServiceAccountsDelete|TestAccResourceGoogleProjectDefaultServiceAccountsDeprivilege|TestAccIAMBetaWorkloadIdentityPoolProvider_iamWorkloadIdentityPoolProviderAwsBasicExample|TestAccIAMBetaWorkloadIdentityPoolProvider_iamWorkloadIdentityPoolProviderAwsFullExample|TestAccIAMBetaWorkloadIdentityPoolProvider_iamWorkloadIdentityPoolProviderOidcBasicExample|TestAccIAMBetaWorkloadIdentityPoolProvider_iamWorkloadIdentityPoolProviderOidcFullExample|TestAccIAMBetaWorkloadIdentityPoolProvider_aws|TestAccIAMBetaWorkloadIdentityPoolProvider_oidc|TestAccOSLoginSSHPublicKey_osLoginSshKeyBasicExample|TestAccStorageBucket_cors You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=155619"