feat(@toss/react):allow null value in SwitchCase component

[haejunejung]

Overview

Refactored the 'SwitchCase' component to accept 'null' as a valid value for 'value' prop, enhancing flexibility and usability

• Updated the 'Props' interface for the 'SwitchCase' component to allow 'value' to be of type 'Case | null'. This change ensures that the component can handle scenarios where 'value' is explicitly set to null.
• Adjusted conditional logic within the 'SwitchCase' function to correctly handle cases where 'value' is null, retruning 'defaultComponent' when appropriate.

PR Checklist

• I read and included theses actions below

1. I have read the Contributing Guide
2. I have written documents and tests, if needed.

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