Fix wrong initialization of maintenance_exclusion

[msgongora]

This PR fixes hashicorp/terraform-provider-google/issues/8112

Release Note Template for Downstream PRs (will be copied)

container: Fix crash due to nil exclusions object when updating an existent cluster with maintenance_policy but without exclusions

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

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@rileykarson, please review this PR or find an appropriate assignee.

[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, 11 insertions(+), 8 deletions(-))
Terraform Beta: Diff ( 2 files changed, 12 insertions(+), 9 deletions(-))

[rileykarson]

LGTM. Thanks @msgongora!