Issue #3458752 by erik_petra: Empty region transform results in error

src/Entity/TransformBlock.php

@@ -231,6 +231,15 @@ class TransformBlock extends ConfigEntityBase implements TransformBlockInterface
     }
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheTagsToInvalidate() {
+    $tags = parent::getCacheTagsToInvalidate();
+    $tags[] = 'config:transform_api.regions';
+    return $tags;
+  }
+
   /**
    * {@inheritdoc}
    */

src/Plugin/Transform/Type/RequestPath.php

@@ -90,7 +90,7 @@ class RequestPath extends TransformationTypeBase {
       foreach ($this->transformBlocks->getVisibleBlocksPerRegion($cacheable_metadata_list)[$region] as $block) {
         $transformation[$block->id()] = BlockTransform::createFromBlock($block);
       }
-      $cacheMetadata = $cacheMetadata->merge($cacheable_metadata_list[$region]);
+      $cacheMetadata = $cacheMetadata->merge($cacheable_metadata_list[$region] ?? new CacheableMetadata());
     }
 
     $cacheMetadata->applyTo($transformation);