[cupertino_ui] Remove two items assert to allow CupertinoTabBar to have one tab

[Wreos]

Based on discussion
Removed assert to enable CupertinoTabBar to support 1 tab and updated the test.

Fixes #42430

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[gemini-code-assist]

Code Review

This pull request updates CupertinoTabBar to allow a single item by changing the assertion from requiring at least two items to requiring at least one, and updates the associated tests and changelog. Feedback suggests using isNotEmpty instead of checking length > 0 on the collection, in line with Effective Dart guidelines.

[gemini-code-assist]

Following the Effective Dart guidelines, it is preferred to use isNotEmpty instead of checking length > 0 on collections.

  }) : assert(items.isNotEmpty, 'CupertinoTabBar requires at least one tab.'),

References

1. Effective Dart recommends using isEmpty and isNotEmpty rather than checking length against 0. ^(link)

[Wreos]

Unfortunately it is not applicable in this case
CupertinoTabBar has a const constructor and items. isNotEmpty is not a valid constant expression.