Do not generate CSS for start and end

[RobinMalfait]

This PR fixes a bug where CSS was generated for start and end. This was accidentally introduced when we moved the start-* and end-* utilities to the legacy utilities. But this meant that we now generate CSS for start and end even if no value is provided.

Fixes: #20002

Test plan

1. Updated the tests to make sure of --spacing: 0.25rem which made the tests fail, and are fixed again by applying the fix.
2. Other tests are still passing

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📒 Files selected for processing (1)

• CHANGELOG.md

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• CHANGELOG.md

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Walkthrough

Tests for legacy start and end utilities were updated to define spacing with a base --spacing: 0.25rem and use multiplier calc(var(--spacing) * N) expressions instead of --spacing-4 tokens. handleInset was simplified to return early when candidate.value === null, removing the previous theme-based fallback and negation logic for missing values. A changelog entry notes that legacy start/end utilities without values should not generate CSS.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely summarizes the main change: preventing CSS generation for start and end utilities without values.
Description check	✅ Passed	The description clearly explains the bug, its cause, the fix, and test validation approach, all related to the changeset.
Linked Issues check	✅ Passed	The PR directly addresses issue #20002 by removing the code path that generates CSS for start/end utilities without values, restoring intended behavior.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the start/end utility generation bug: test updates, implementation fix, and changelog entry.

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