chore: document $breakpoint variable in xy-grid-layout, set null as default for $breakpoint

Author: Daniel Ruf

scss/xy-grid/_layout.scss

@@ -14,6 +14,7 @@
 /// @param {Number|Map} $gutters [$grid-margin-gutters] - Map or single value for gutters.
 /// @param {Keyword} $gutter-type [margin] - Type of gutter to output. Accepts `margin` or `padding`.
 /// @param {List} $gutter-position [right left] - The position to apply gutters to. Accepts `top`, `bottom`, `left`, `right` in any combination.
+/// @param {String} $breakpoint [null] - The breakpoint to use for the cell generation.
 /// @param {Boolean} $vertical [false] - Set to true to output vertical (height) styles rather than widths.
 @mixin xy-grid-layout(
   $n,
@@ -22,7 +23,7 @@
   $gutters: $grid-margin-gutters,
   $gutter-type: margin,
   $gutter-position: right left,
-  $breakpoint: $-zf-zero-breakpoint,
+  $breakpoint: null,
   $vertical: false
 ) {
   $size: percentage(1/$n);