Do not give “false ID” to invoice products that come from quotations

When adding “free-form products” to quotes they do not have a product
ID, but i has coalescing the NULL to zero because product_id is an
integer and can not coalesce a nullable integer to an empty string.

However, that causes problems when trying to create the invoice for that
quote, because it tries to add products that have an ID of 0 and the
foreign key, obviously, fail.

At first i modified NewInvoiceProductArray.EncodeBinary to check for
"0" as well as the empty string, but i realized this was wrong: the
problem was because i gave these products an ID when they do not have
any.  And the solution is to cast product_id to a text, which is what
will get converted anyway because i the only thing i do to it is to
store to a string-backed InputForm field.

Closes #73.

pkg/invoices.go

@@ -958,7 +958,7 @@ func (form *invoiceForm) MustFillFromQuote(ctx context.Context, conn *Conn, slug
 		return false
 	}
 	form.Products = []*invoiceProductForm{}
-	form.mustAddProductsFromQuery(ctx, conn, "select '', coalesce(product_id, 0), name, description, to_price(price, $2), quantity, (discount_rate * 100)::integer, array_remove(array_agg(tax_id), null) from quote_product left join quote_product_product using (quote_product_id) left join quote_product_tax using (quote_product_id) where quote_id = $1 group by quote_product_id, coalesce(product_id, 0), name, description, discount_rate, price, quantity", quoteId, form.company.DecimalDigits)
+	form.mustAddProductsFromQuery(ctx, conn, "select '', coalesce(product_id::text, ''), name, description, to_price(price, $2), quantity, (discount_rate * 100)::integer, array_remove(array_agg(tax_id), null) from quote_product left join quote_product_product using (quote_product_id) left join quote_product_tax using (quote_product_id) where quote_id = $1 group by quote_product_id, coalesce(product_id::text, ''), name, description, discount_rate, price, quantity", quoteId, form.company.DecimalDigits)
 	return true
 }