Tableau : How to replace NULL string with blank or some string

//Method - 1

//if ISNULL([Prerequisite]) then INT('')
//ELSE [Prerequisite]
//END
//Method -2

IFNULL(str([Prerequisite]),'NA')

//Method -3

//if (ISNULL([Prerequisite])) then ' '
//ELSE STR([Prerequisite])
//END