Built-in conversion functions convert NULL to default(T) instead of returning NULL

[dallmair]

At the moment, every ToXxx method in BuildInFunctions returns the Xxx type's default value. This is somewhat understandable for the integer types, but TO_BOOLEAN(NULL) returning FALSE is hard to understand for users, and with TO_DATETIME(NULL) returning 0001-01-01 00:00:00 it's almost impossible to convince users that this behavior is intended (though of course it obviously is, because the code is currently explicitly written to perform this way).

From my point of view, these conversion functions should be changed to return NULL instead. When #30 is done, this could be as simple as not handling the NULL values manually anymore.

[dallmair]

Works fine in v.Next.