Fix: Handle invalid dates in serialization

[trentonallan]

Fixes #3318

Problem

new Date(undefined) and other invalid dates were being serialized as "0NaN-NaN-NaNTNaN:NaN:NaN.NaN+NaN:NaN" instead of being handled properly.

Solution

Added validation in dateToString and dateToStringUTC functions to check if date is valid before serialization using isNaN(date.getTime()). Invalid dates now throw a TypeError with a clear error message.

Testing

• Added unit tests to verify invalid dates throw errors
• Added test to ensure valid dates still work correctly
• All existing tests pass

[charmander]

waste of maintainer time with incorrect AI-generated pull request, exhibit n

A correct implementation is already available in #3330.