Add zero fallback to sum() calls

If there are no data, sum() doesn't return zero but null.
Passing null to the internal PHP functions causes issues
and falling back to "zero" seems like a good option in
all of the affected places.

remp/crm#2706

src/Models/Generator/templates/invoice/default.latte

@@ -277,7 +277,7 @@
     <hr>
 
     {php ksort($vatRates)}
-    {var $totalPrice = $invoice->related('invoice_items')->sum('price * count')}
+    {var $totalPrice = $invoice->related('invoice_items')->sum('price * count') ?? 0.00}
 
     <!-- notes -->
     <div class="cell-xs-4 float">
@@ -307,8 +307,8 @@
             <tbody>
             {foreach $vatRates as $rate => $_}
                 <tr>
-                    {var $itemsPrice = $invoice->related('invoice_items')->where('vat', $rate)->sum('price * count')}
-                    {var $itemsPriceWithoutVat = $invoice->related('invoice_items')->where('vat', $rate)->sum('price_without_vat * count')}
+                    {var $itemsPrice = $invoice->related('invoice_items')->where('vat', $rate)->sum('price * count') ?? 0.00}
+                    {var $itemsPriceWithoutVat = $invoice->related('invoice_items')->where('vat', $rate)->sum('price_without_vat * count') ?? 0.00}
                     {var $vatAmount = $itemsPrice - $itemsPriceWithoutVat}
 
                     <td>{$rate}%</td>