Use a null as product ID when adding new products to invoices

Otherwise, pgx (rightfully) tries to convert a "" into a integer, as
this is the field’s type, cannot, and panics with an error.

Added a IntegerOrNull method to FormField because this is exactly the
same that happens with the invoiceProductId, and made no sense to have
to do the logic twice, or in a function inside form.

pkg/form.go

@@ -70,6 +70,15 @@ func (field *InputField) Integer() int {
 	return value
 }
 
+func (field *InputField) IntegerOrNil() interface{} {
+	if field.Val != "" {
+		if i := field.Integer(); i > 0 {
+			return i
+		}
+	}
+	return nil
+}
+
 func (field *InputField) Float64() float64 {
 	value, err := strconv.ParseFloat(field.Val, 64)
 	if err != nil {

pkg/pgtypes.go

@@ -48,15 +48,9 @@ func (src EditedInvoiceProductArray) EncodeBinary(ci *pgtype.ConnInfo, buf []byt
 	}
 	var values [][]interface{}
 	for _, form := range src {
-		var invoiceProductId interface{} = nil
-		if form.InvoiceProductId.Val != "" {
-			if id := form.InvoiceProductId.Integer(); id > 0 {
-				invoiceProductId = id
-			}
-		}
 		values = append(values, []interface{}{
-			invoiceProductId,
+			form.InvoiceProductId.IntegerOrNil(),
-			form.ProductId.Val,
+			form.ProductId.IntegerOrNil(),
 			form.Name.Val,
 			form.Description.Val,
 			form.Price.Val,