Fixed Input Unit Block: Unit jumps to px when set to 0

[Mayank-Tripathi32]

fixes #51616

What?

This PR fixes the issue where the unit in the spacer and dimension controls resets to px when the input value is cleared.

Why?

When removing the value from input fields like spacer height or padding in the dimensions control, the unit automatically resets to px. This behavior causes unexpected results and reduces user control over custom styles. Fixing this improves usability and ensures a consistent editing experience.

How?

The fix was applied in packages/components/src/unit-control/index.tsx, where the unit was being dropped when the input value was cleared. The implementation now ensures that when the value is empty, it defaults to 0 with the current unit preserved, preventing the reset to px.

Testing Instructions

1. Open the WordPress editor (post or page).
2. Add a Spacer block inside a stack.
3. Change its unit to vh.
4. Clear the height value to input a new value.
5. Ensure the unit stays as vh and does not revert to px.

Additional Test:

1. Go to Global Styles settings.
2. Select the Button block.
3. Adjust padding values and clear the input fields.
4. Ensure the unit remains consistent and doesn’t default to px.

Screencast

test-unit-controls.mp4

[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: Mayank-Tripathi32 <mayanktripathi32@git.wordpress.org>
Co-authored-by: ndiego <ndiego@git.wordpress.org>
Co-authored-by: hanneslsm <hanneslsm@git.wordpress.org>

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

[Mayank-Tripathi32]

If the proposed changes seem correct, I can update the failing unit tests in each component to handle value instead of undefined. This adjustment will align the tests with the updated behavior and ensure they pass as expected.

[Mayank-Tripathi32]

If the proposed changes seem correct, I can update the failing unit tests in each component to handle value instead of undefined. This adjustment will align the tests with the updated behavior and ensure they pass as expected.

It seems the test cases expects the default behaviour of "undefined", So I think its better to introduce a prop "shouldPreserveUnit" and conditionally add it for required places.