Filter out nulls in tax array when viewing invoice

This is for the unusual case of products without taxes, that PostgreSQL
by default would generate an array with a single null value in it, but
then pgx would not be able to set that null to the string variable.

pkg/invoices.go

@@ -336,7 +336,7 @@ func mustGetInvoice(ctx context.Context, conn *Conn, company *Company, slug stri
 	if err := conn.QueryRow(ctx, "select array_agg(array[name, to_price(amount, $2)]) from invoice_tax_amount join tax using (tax_id) where invoice_id = $1", invoiceId, decimalDigits).Scan(&inv.Taxes); err != nil {
 		panic(err)
 	}
-	rows := conn.MustQuery(ctx, "select invoice_product.name, description, to_price(price, $2), (discount_rate * 100)::integer, quantity, to_price(subtotal, $2), to_price(total, $2), array_agg(array[tax_class.name, (tax_rate * 100)::integer::text]) from invoice_product join invoice_product_amount using (invoice_product_id) left join invoice_product_tax using (invoice_product_id) left join tax using (tax_id) left join tax_class using (tax_class_id) where invoice_id = $1 group by invoice_product.name, description, discount_rate, price, quantity, subtotal, total", invoiceId, decimalDigits)
+	rows := conn.MustQuery(ctx, "select invoice_product.name, description, to_price(price, $2), (discount_rate * 100)::integer, quantity, to_price(subtotal, $2), to_price(total, $2), array_agg(array[tax_class.name, (tax_rate * 100)::integer::text]) filter (where tax_rate is not null) from invoice_product join invoice_product_amount using (invoice_product_id) left join invoice_product_tax using (invoice_product_id) left join tax using (tax_id) left join tax_class using (tax_class_id) where invoice_id = $1 group by invoice_product.name, description, discount_rate, price, quantity, subtotal, total", invoiceId, decimalDigits)
 	defer rows.Close()
 	taxClasses := map[string]bool{}
 	for rows.Next() {