waf/helpers: prevent panic during ExpandActivatedRule method

[anGie44]

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Closes #22500
Closes #22378

Output from acceptance testing:

--- PASS: TestAccWAFRuleGroup_noActivatedRules (24.10s)
--- PASS: TestAccWAFRuleGroup_disappears (32.71s)
--- PASS: TestAccWAFRuleGroup_basic (33.40s)
--- PASS: TestAccWAFRuleGroup_tags (52.01s)
--- PASS: TestAccWAFRuleGroup_changeActivatedRules (58.68s)
--- PASS: TestAccWAFRuleGroup_changeNameForceNew (59.35s)

--- PASS: TestAccWAFRegionalRuleGroup_noActivatedRules (29.69s)
--- PASS: TestAccWAFRegionalRuleGroup_disappears (43.75s)
--- PASS: TestAccWAFRegionalRuleGroup_basic (48.69s)
--- PASS: TestAccWAFRegionalRuleGroup_changeActivatedRules (68.40s)
--- PASS: TestAccWAFRegionalRuleGroup_changeNameForceNew (71.84s)
--- PASS: TestAccWAFRegionalRuleGroup_tags (80.44s)

[ewbankkit]

LGTM 🚀.

[github-actions]

This functionality has been released in v4.0.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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