database/sql: add generic Null[T]

[methane]

database/sql doesn't have NullUInt64. So drivers and ORMs may implement their own NullUInt64.
Application developers would be confused which NullUInt64 is returned by dynamic Scan methods.

When NullUInt64 was proposed in past, @a8m, @kardianos, @rsc said "wait for generic".
#47953 (comment)

Now we have generics. So can we have it for now? Should it Null[T] or Nullable[T]?

Here is quick implementation: https://go.dev/play/p/Jtx8hP0H1_Z

[seankhliao]

see also #48702

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

/cc @kardianos @bradfitz

[rsc]

Looking at the existing NullInt64, etc they name the value field after the type (Int64 for NullInt64). We can't do that for the generic version - it needs a single field name for all the instances. Value is the obvious choice but that has to be a method. Val is the next obvious choice but then the fields are Val and Valid. Maybe X and Valid?

[rsc]

Does someone want to write a CL and see how it looks?

[a8m]

Maybe X and Valid?

I suggest going with V and Valid, and don’t mind changing my CL to support this.

but that has to be a method

Could you please elaborate on why it needs to be a method?