Fix definition of global variables

[LeBenLeBen]

It currently triggers deprecation warnings in the latest Dart Sass version:

DEPRECATION WARNING: As of Dart Sass 2.0.0, !global assignments won't be able to
declare new variables. Consider adding `$spacing-value: null` at the top level.

   ╷
13 │     $spacing-value: nth($spacing, 2) !global;
   │     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   ╵
    node_modules/kanbasu/src/scss/tools/_mixins.scss 13:5    foreach-alternative-spacing()

Hint from their Changelog:

Deprecate !global variable assignments to variables that aren't yet defined. This deprecation message can be avoided by assigning variables to null at the top level before globally assigning values to them.

[LeBenLeBen]

Changed since 2.5.0-beta.1