Avoid null value (#27570)

scss/mixins/_breakpoints.scss

@@ -16,7 +16,7 @@
 //    md
 @function breakpoint-next($name, $breakpoints: $grid-breakpoints, $breakpoint-names: map-keys($breakpoints)) {
   $n: index($breakpoint-names, $name);
-  @return if($n < length($breakpoint-names), nth($breakpoint-names, $n + 1), null);
+  @return if($n != null and $n < length($breakpoint-names), nth($breakpoint-names, $n + 1), null);
 }
 
 // Minimum breakpoint width. Null for the smallest (first) breakpoint.