Fixes at promotion logic

src/Promotion/Actions/CartFixedDiscount.php

@@ -40,6 +40,7 @@ public function apply(object $subject, array $configuration): array
         if ($subject instanceof Adjustable) {
             $result[] = $subject->adjustments()->create($this->getAdjuster($configuration));
         }
+        //@todo also set the origin
 
         return $result;
     }

src/Promotion/Models/Coupon.php

@@ -51,6 +51,10 @@ public function canBeUsed(): bool
 
     public function isExpired(): bool
     {
+        if (null === $this->expires_at) {
+            return false;
+        }
+
         return $this->expires_at->isPast();
     }
 

src/Promotion/Models/Promotion.php

@@ -92,7 +92,7 @@ public function getActions(): Collection
 
     public function isValid(?\DateTimeInterface $at = null): bool
     {
-        if ($this->usage_count >= $this->usage_limit) {
+        if (null !== $this->usage_limit && $this->usage_count >= $this->usage_limit) {
             return false;
         }