CREATE PROCEDURE dbo.[mainsproc]
  WITH EXECUTE AS 'dynamicdatauser'
AS
BEGIN
  DECLARE @SPROCNAME NVARCHAR(1000) = N'dbo.[optionalsproc]'
  IF OBJECT_ID(@SPROCNAME,N'P') IS NOT NULL THEN
  BEGIN
    DECLARE @DYNSQL NVARCHAR(100) = N'EXEC ' + @SPROCNAME
    EXEC sp_executeSQL @DYNSQL
  END
END