Fix result type decl for nullable :one queries

[jschaf]

Previously, if a query returned a single nullable column, pggen would return the correct type *string, but only because we took the address of the input.

Bad code:

var item string
if err := row.Scan(&item); err != nil {
	return &item, fmt.Errorf("query StringAggFirstName: %w", err)
}
return &item, nil

Fixed code:

var item *string
if err := row.Scan(&item); err != nil {
	return item, fmt.Errorf("query StringAggFirstName: %w", err)
}
return item, nil

Fixes #75.