Allow to leave amount field empty when adding a new part lot.

src/Entity/Parts/PartLot.php

@@ -303,8 +303,20 @@ class PartLot extends AbstractDBElement implements TimeStampableInterface, Named
         return (float) $this->amount;
     }
 
-    public function setAmount(float $new_amount): self
+    /**
+     * Sets the amount of parts in the part lot.
+     * If null is passed, amount will be set to unknown.
+     * @param  float|null  $new_amount
+     * @return $this
+     */
+    public function setAmount(?float $new_amount): self
     {
+        //Treat null like unknown amount
+        if ($new_amount === null) {
+            $this->instock_unknown = true;
+            $new_amount = 0.0;
+        }
+
         $this->amount = $new_amount;
 
         return $this;

src/Form/Part/PartLotType.php

@@ -88,6 +88,7 @@ class PartLotType extends AbstractType
 
         $builder->add('amount', SIUnitType::class, [
             'measurement_unit' => $options['measurement_unit'],
+            'required' => false,
             'label' => 'part_lot.edit.amount',
             'attr' => [
                 'class' => 'form-control-sm',