Return nil parent_tree_url for root tree entity

app/serializers/tree_root_entity.rb

 # TODO: Inherit from TreeEntity, when `Tree` implements `id` and `name` like `Gitlab::Git::Tree`.
 class TreeRootEntity < Grape::Entity
   include RequestAwareEntity
-  
+
   expose :path
 
   expose :trees, using: TreeEntity
@@ -9,9 +9,10 @@ class TreeRootEntity < Grape::Entity
   expose :submodules, using: SubmoduleEntity
 
   expose :parent_tree_url do |tree|
-    next unless tree.path.present?
+    path = tree.path.sub(%r{\A/}, '')
+    next unless path.present?
 
-    path_segments = tree.path.split('/')
+    path_segments = path.split('/')
     path_segments.pop
     parent_tree_path = path_segments.join('/')