For what its worth, COALESCE is similiar but
is the exact match you're looking for in DB2.
COALESCE allows multiple arguments, returning the first NON NULL expression, whereas IFNULL only permits the expression and the default.
SELECT product.ID, IFNULL(product.Name, "Internal") AS ProductName
Gives you what you're looking for as well as the previous answers, just adding for completeness.