fix: set default value of schedule attribute of aws_backup_plan rule

[pdecat]

Rollback Plan

If a change needs to be reverted, we will publish an updated version of the library.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

Description

ScheduleExpression
A CRON expression in UTC specifying when AWS Backup initiates a backup job. When no CRON expression is provided, AWS Backup will use the default expression cron(0 5 ? * * *).

https://docs.aws.amazon.com/aws-backup/latest/devguide/API_BackupRuleInput.html#Backup-Type-BackupRuleInput-ScheduleExpression

Relations

Closes #23976

References

Output from Acceptance Testing

% make testacc PKG=backup TESTS=TestAccBackupPlan_no_schedule
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
make: Running acceptance tests on branch: 🌿 b-backup_plan_schedule_computed 🌿...
TF_ACC=1 go1.24.10 test ./internal/service/backup/... -v -count 1 -parallel 20 -run='TestAccBackupPlan_no_schedule'  -timeout 360m -vet=off
2025/12/02 17:36:41 Creating Terraform AWS Provider (SDKv2-style)...
2025/12/02 17:36:41 Initializing Terraform AWS Provider (SDKv2-style)...
=== RUN   TestAccBackupPlan_no_schedule
=== PAUSE TestAccBackupPlan_no_schedule
=== CONT  TestAccBackupPlan_no_schedule
--- PASS: TestAccBackupPlan_no_schedule (26.68s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/backup     26.787s

[github-actions]

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[github-actions]

✅ Thank you for correcting the previously detected issues! The maintainers appreciate your efforts to make the review process as smooth as possible.