Issue #3465432 by pdureau, Grimreaper: [2.0.0-beta1] empty variants in pattern() Twig function

Closes #3465432

modules/ui_patterns_legacy/src/Template/TwigExtension.php

@@ -56,8 +56,14 @@ class TwigExtension extends AbstractExtension {
     $element = [
       '#type' => 'pattern',
       '#id' => $id,
-      '#variant' => $variant,
     ];
+
+    // Variant is a JSON-schema enum where empty string is not always allowed.
+    // Moreover, we don't allow empty variant.
+    if (!empty($variant)) {
+      $element['#variant'] = $variant;
+    }
+
     // We use the compact format, so it will be resolved by
     // RenderableConverter::resolveCompactFormat() before being converted to
     // the SDC format.