add default null to Instance entity nullables field

add default values of null to DB nullable properties to prevent
access before initialization problems on php side

currently observed the error with the field
`Instance::$lastSuccessfulReceive` but added null default to all
nullable fields just in case

    "Handling "App\Message\ActivityPub\Inbox\ActivityMessage" failed:
    Typed property App\Entity\Instance::$lastSuccessfulReceive must not
    be accessed before initialization"

src/Entity/Instance.php

@@ -27,22 +27,22 @@ public static function getDateBeforeDead(): \DateTimeImmutable
     }
 
     #[Column(nullable: true)]
-    public ?string $software;
+    public ?string $software = null;
 
     #[Column(nullable: true)]
-    public ?string $version;
+    public ?string $version = null;
 
     #[Column(unique: true)]
     public string $domain;
 
     #[Column(type: 'datetimetz_immutable', nullable: true)]
-    private ?\DateTimeImmutable $lastSuccessfulDeliver;
+    private ?\DateTimeImmutable $lastSuccessfulDeliver = null;
 
     #[Column(type: 'datetimetz_immutable', nullable: true)]
-    private ?\DateTimeImmutable $lastFailedDeliver;
+    private ?\DateTimeImmutable $lastFailedDeliver = null;
 
     #[Column(type: 'datetimetz_immutable', nullable: true)]
-    private ?\DateTimeImmutable $lastSuccessfulReceive;
+    private ?\DateTimeImmutable $lastSuccessfulReceive = null;
 
     #[Column]
     private int $failedDelivers = 0;