Fix nil panic in container cluster filtered notification config

[zli82016]

fixes hashicorp/terraform-provider-google#26573

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
google-beta provider: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))

[modular-magician]

Tests analytics

Total tests: 284
Passed tests: 269
Skipped tests: 15
Affected tests: 0

Click here to see the affected service packages

• container

🟢 All tests passed!

View the build log

[zli82016]

@modular-magician reassign-reviewer

[github-actions]

Googlers: For automatic test runs see go/terraform-auto-test-runs.

@SirGitsalot, a repository maintainer, has been assigned to review your changes. If you have not received review feedback within 2 business days, please leave a comment on this PR asking them to take a look.

You can help make sure that review is quick by doing a self-review and by running impacted tests locally.

[github-actions]

@SirGitsalot This PR has been waiting for review for 3 weekdays. Please take a look! Use the label disable-review-reminders to disable these notifications.

[SirGitsalot]

Sorry the delay!