support empty string in breakpoint() mixin

modules/primer-support/lib/mixins/layout.scss

@@ -1,21 +1,27 @@
 // Responsive media queries
 
 @mixin breakpoint($breakpoint) {
-  // Retrieves the value from the key
-  $value: map-get($breakpoints, $breakpoint);
-
-  // If the key exists in the map
-  @if $value != null {
-    // Prints a media query based on the value
-    @media (min-width: $value) {
-      @content;
-    }
+  @if $breakpoint == "" {
+    @content;
   }
 
-  // If the key doesn't exist in the map
   @else {
-    @warn "Unfortunately, no value could be retrieved from `#{$breakpoint}`. "
-      + "Please make sure it is defined in `$breakpoints` map.";
+    // Retrieves the value from the key
+    $value: map-get($breakpoints, $breakpoint);
+
+    // If the key exists in the map
+    @if $value != null {
+      // Prints a media query based on the value
+      @media (min-width: $value) {
+        @content;
+      }
+    }
+
+    // If the key doesn't exist in the map
+    @else {
+      @warn "Unfortunately, no value could be retrieved from `#{$breakpoint}`. "
+        + "Please make sure it is defined in `$breakpoints` map.";
+    }
   }
 }