Do not generate CSS for `start` and `end` (#20003)

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