Handling nil valuers.

[ilmaruk]

I took the liberty of creating this PR, because we're having troubles with nil pointers to types that implement driver.Valuer. In out case it's uuid.UUID, where:

func Foo(ctx context.Context, runner dbr.SessionRunner, id *uuid.UUID) error {
    stmt := runner.InsertInto(table).Pair("id", id)
}

would panic if id is nil.