IF statement inside SQL Function

I need to do something like this but I can't manage to find the correct syntax.

ALTER function [dbo].[FN_ObtenerArbolEmpresa]    (@empresaID int)
returns table 
as
BEGIN
IF (@empresaID = 0)
  begin
    RETURN select id_empresa_hijo from empresa_x_empresa
  end
ELSE
  begin
  return 
  (
  WITH  temp
      AS (
            -- anchor
          SELECT  id_empresa_hijo, id_empresa_padre
          FROM    empresa_x_empresa with(nolock)
          WHERE   id_empresa_padre = @empresaID
          UNION ALL
            --recursive member
          SELECT  t.id_empresa_hijo, t.id_empresa_padre
          FROM    empresa_x_empresa AS t with(nolock)
                  JOIN temp AS a
                    ON t.id_empresa_padre = a.id_empresa_hijo
         )
  (
  SELECT id_empresa_hijo  FROM temp 
  union 
  select @empresaID
  )
 end

Actually I just want to avoid the recursion when @empresaID = 0.