fix(v5): `locale` type can be `null`

[olrtg]

Types of changes

• [x] Bug fix (a non-breaking change which fixes an issue)
• [ ] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description

This PR fixes the type of the locale property for the v5 composable. Part of #434.

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes (if not applicable, please state why)

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