Do not allow mutation for empty clauses

activerecord/lib/active_record/relation/from_clause.rb

@@ -19,7 +19,7 @@ module ActiveRecord
       end
 
       def self.empty
-        @empty ||= new(nil, nil)
+        @empty ||= new(nil, nil).freeze
       end
     end
   end

activerecord/lib/active_record/relation/where_clause.rb

@@ -83,7 +83,7 @@ module ActiveRecord
       end
 
       def self.empty
-        @empty ||= new([])
+        @empty ||= new([]).tap(&:referenced_columns).freeze
       end
 
       def contradiction?