Components: Add fallback units for input slider in BoxControl

[im3dabasia]

What?

Closes #61410

In trunk, the selected units are undefined initially when the component is mounted. Due to this, when the slider is used, the values are updated without any unit. To solve this, a fallback value has been added in case the unit is undefined.

Why?

Solves a bug where the input slider of box control by default does not have any units attached to the number values.

How?

Added fallback logic to ensure units are always defined.

Testing Instructions

1. Run npm run storybook:dev
2. Checkout the BoxControl component
3. Confirm that the slider has both value and unit when changed.

Screenshots or screencast

Before

Screen.Recording.2025-06-02.at.2.40.05.PM.mov

After

Screen.Recording.2025-06-02.at.2.40.05.PM.mov

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