DMN17 — date() conversion function should return null for invalid dates

The description of the date() conversion function doesn't specify the expected result if the function is invoked with an invalid date.

Examples:

date("2023-02-29") 
// expected: null (2023 is not a leap year)

date("2023-06-31")
// expected: null (June has only 30 days)

I expect that the function should return null because an invalid date is outside of the parameter domain of a "valid date".

From a user perspective, it would also be useful to return null to identify/detect invalid dates.

Goal: clarify the expected behavior and adjust the description in the specification.