fix: Numeric value clearing in preset input controls

[DarkMatter-999]

What?

Closes #77124

Fixes a bug where clearing numeric values in shared preset-based controls (used by inputs like margin and border radius) could retain a partial value (for example, 6 after deleting 60).

Why?

When users clear numeric values with backspace in affected block controls, the intermediate partial value may be applied and persisted instead of the field becoming fully empty. This creates incorrect style values and inconsistent UX.

How?

Updates PresetInputControl custom value change handling so empty input is treated as a valid cleared state instead of being discarded as undefined. This allows the control to propagate a true empty value and avoids persisting transient partial numeric states during deletion.

Testing Instructions

Testing Instructions for Keyboard

1. Open the editor and insert a block (for example, Paragraph).
2. In block settings, set a numeric value like 60 in controls backed by preset input (e.g. Margin, Padding).
3. Place the cursor in the numeric field and press backspace until cleared.
4. Confirm the value is fully cleared (empty), and no partial value like 6 is retained.
5. Repeat for at least one additional affected control to confirm shared behavior.

Screenshots or screencast

Before

number-input.mov

After

number-input-fix.mov

Use of AI Tools

I used GitHub Copilot as an assistive drafting and code-review aid for the PR.

[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @DarkMatter-999! In case you missed it, we'd love to have you join us in our Slack community.

If you want to learn more about WordPress development in general, check out the Core Handbook full of helpful information.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: DarkMatter-999 <lakshyajeet@git.wordpress.org>
Co-authored-by: dpmehta <mehtadev@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.