Normalize `decl.before` and `decl.between`

`decl.before` and `decl.between` can be `undefined`.

Also can be `undefined` following properties:

  - `comment.before`
  - `decl.important`
  - `rule.before`
  - `rule.between`
  - `rule.semicolon`
  - `rule.after`
  - `atRule.before`
  - `atRule.afterName`
  - `atRule.between`
  - `atRule.semicolon`
  - `atRule.after`

But these properties reset correctly.