fix: fix typo with CSS var in mixin

Author: bolt-bot

packages/uikit-workshop/src/sass/scss/01-abstracts/_mixins.scss

@@ -101,9 +101,8 @@
   */
   &.pl-is-active {
     max-height: calc(100vh - #{$offset-top} - 1rem);
-    max-height: var(
-      calc(--pl-viewport-height - 1rem),
-      calc(100vh - #{$offset-top} - 1rem)
+    max-height: calc(
+      var(--pl-viewport-height, calc(100vh - #{$offset-top})) - 1rem
     ); /* [1] */
     overflow: auto;
     -webkit-overflow-scrolling: touch;