Avoid shape-related performance warning for Sequel::SQL::StringAgg

Making the order the same helps.  This does define an unnecessary
instance variable, but it makes the order consistent.  This keeps
the object to 2 shapes (one without @order_expr and @distinct,
and one with both).

lib/sequel/extensions/string_agg.rb

@@ -173,16 +173,16 @@ def is_distinct?
       # Return a modified StringAgg that uses distinct expressions
       def distinct
         self.class.new(@expr, @separator) do |sa|
-          sa.instance_variable_set(:@order_expr, @order_expr) if @order_expr
+          sa.instance_variable_set(:@order_expr, @order_expr)
           sa.instance_variable_set(:@distinct, true)
         end
       end
 
       # Return a modified StringAgg with the given order
       def order(*o)
         self.class.new(@expr, @separator) do |sa|
-          sa.instance_variable_set(:@distinct, @distinct) if @distinct
           sa.instance_variable_set(:@order_expr, o.empty? ? nil : o.freeze)
+          sa.instance_variable_set(:@distinct, @distinct)
         end
       end