[preset-mini] Cannot read properties of undefined (reading 'split') when overriding spacing config

[1001v]

UnoCSS version

0.58.6

Describe the bug

Hello. Preset Mini throws an exception "Cannot read properties of undefined (reading 'split')" when we override spacing field in the theme completely and miss the DEFAULT property.
It's highly likely, that Uno will try to generate a corresponding class for the px (which is equal to px-DEFAULT) entry against developer's will, since px is also a shortcut for pixel, and the presence of this entry outside the CSS classes context is kind of expected.
I see, how keeping defaults is a good practice. However, this behavior, in my opinion, in unexpected. Thanks.

Reproduction

https://unocss.dev/play/?html=DwEwlgbgfARADgDxsA9OaQ&config=PTAEFMGcBsEsDsAuBaAJrSBDARtcz5wAPFOQ0eAe2QCcpEbYBjRcVZWAWwAdKbFIAKC69%2BoAN6DQoVOABmCcAGFK8BQHMANFNDc6kcIgCCiBrGwBXRLDkBPbdL1RDASSarID3fsMBVKtoAvqByNJScoADkFlRMkJCRgoLEoogy8pgW0GmyCoQqarDqABSS0jRZUABcoADaOtK1kUwWkIjhtJWRmhKg7tB8NZF0qJGggQC6XlM6kAAWfIgtAjVl0lEtbR3zi8uRQ6wkyNDqoIcofJjw6uCgCwBu4DRV58ismNDdOoFeTgYrdQa3mciH8lGKAEovI4fMZTIxLNY7JDocD-m4PKUgdJIEwPuAagBGAB0ACZUdImKh4EM5qZuJAqiAoJxifNgF91uMIToZtJiKx4KgACpzcCcAmgYqIMUSiGgAC8AD4JECZeLwGzuJgmAhTgrepwhgAObhEMaBb6CQI8oA&css=PQKgBA6gTglgLgUzAYwK4Gc4HsC2YDCAyoWABYJQIA0YAhgHYAmYcUD6AZllDhWOqgAOg7nAB0YAGLcwCAB60cggDYIAXGBDAAUKDBi0mXGADe2sGC704AWgDuCGAHNScDQFYADJ4Dc5sAACtMLKAJ5gggCMLPK2ABR2pPBIcsoAlH4WAEa0yADWTlBYqEw2yFjK3Bpw5LxxAOTllVDoYpSMYgAs3vUZ2gC%2BmsBAA&options=N4XyA

System Info

No response

Validations

• Read the Contributing Guidelines.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.