fixes [Showcase `Badge` widget in `NavigationBar` and `NavigationRail` examples
](https://github.com/flutter/flutter/issues/129832)

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| <img src="https://github.com/flutter/flutter/assets/48603081/808c9577-c6b4-465f-b9fe-100d422dd408" /> | <img src="https://github.com/flutter/flutter/assets/48603081/c9b3ee03-56d7-4220-94cf-06e235631714" /> | <img src="https://github.com/flutter/flutter/assets/48603081/43fab47b-25e8-4412-92d2-6d4868e43ff8"  /> |