sql - how to find if incomming parameter in not null and not empty in Sybase -

I'm trying to write something like this in CBS ASE

  @searchstring Varchar (500) If the best way to check this condition is INULLorEpty for the incoming @ parameter (ANNLLERTE (@sizerstring)) {exec (@strexecsql)} and {exec (@ strexecsql1)}  < 
 
 
 
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 Best would be ISNULL, that is: 
 
  DECLARE @ SearchString varchar (500 ) ... if (ISNULL (@searchstring, '') = '') EXEC (@strexecsql) ELSE EXEC (@ strexecsql1)