Mark shasum and reference property of Dist as nullable

This is the case in, for example, https://packagist.org/packages/roots/wordpress.json

src/Packagist/Api/Result/Package/Dist.php

@@ -8,15 +8,15 @@
 
 class Dist extends AbstractResult
 {
-    protected string $shasum;
+    protected ?string $shasum;
 
     protected string $type;
 
     protected string $url;
 
-    protected string $reference;
+    protected ?string $reference;
 
-    public function getShasum(): string
+    public function getShasum(): ?string
     {
         return $this->shasum;
     }
@@ -31,7 +31,7 @@ public function getUrl(): string
         return $this->url;
     }
 
-    public function getReference(): string
+    public function getReference(): ?string
     {
         return $this->reference;
     }