fix negative allocation

Author: QuentinGab

src/Pdf/PdfInvoice.php

@@ -137,7 +137,7 @@ public function totalTaxAmount(): Money
          * we allocate the discount across items before computing taxes.
          */
         $allocatedDiscounts = $totalDiscount->allocate(...array_map(
-            fn ($item) => $item->subTotalAmount()->getMinorAmount()->toInt(),
+            fn ($item) => $item->subTotalAmount()->abs()->getMinorAmount()->toInt(),
             $this->items
         ));
 

tests/Feature/PdfInvoiceTest.php

@@ -43,4 +43,6 @@
     [[100.0, 50.0], 0.0, 0.0, 150.0, 0.0, 0.0, 150.0],
     [[100.0, 50.0], 20.0, 0.0, 150.0, 0.0, 30.0, 180.0],
     [[100.0, 50.0], 20.0, 10.0, 150.0, 15.0, 27.0, 162.0],
+    [[-100.0], 0.0, 0.0, -100.0, 0.0, 0.0, -100.0],
+    [[-100.0, -50.0], 20.0, 10.0, -150.0, -15.0, -27.0, -162.0],
 ]);