fix: types of the extend variants

[tianenpang]

Fixes https://github.com/nextui-org/tailwind-variants/issues/15 Fixes https://github.com/nextui-org/tailwind-variants/issues/44

Description

Fixed types of the extend variants by asserting slots and providing default value ({}).

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What is the purpose of this pull request?

• [x] Bug fix
• [ ] New Feature
• [ ] Documentation update
• [ ] Other

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