restore removed scratch asset validations

Author: rammodhvadia

app/models/scratch_asset.rb

@@ -5,6 +5,8 @@ class ScratchAsset < ApplicationRecord
   has_one_attached :file
 
   validates :filename, presence: true, uniqueness: { scope: %i[project_id uploaded_user_id] }
+  validates :uploaded_user_id, absence: true, if: :global?
+  validates :uploaded_user_id, presence: true, unless: :global?
   validate :belongs_to_scratch_project
 
   scope :global_assets, -> { where(project_id: nil, uploaded_user_id: nil) }