fix(decision): inconsistency in (sub)decision model with GEWISDB

Somehow the possibility for these to be `null` got lost somewhere.

module/Decision/src/Model/Decision.php

@@ -6,6 +6,7 @@
 
 use Decision\Model\Enums\MeetingTypes;
 use Decision\Model\SubDecision\Annulment;
+use Doctrine\Common\Collections\ArrayCollection;
 use Doctrine\Common\Collections\Collection;
 use Doctrine\ORM\Mapping\Column;
 use Doctrine\ORM\Mapping\Entity;
@@ -52,6 +53,7 @@ class Decision
         enumType: MeetingTypes::class,
     )]
     protected MeetingTypes $meeting_type;
+
     /**
      * Meeting number.
      *
@@ -103,7 +105,12 @@ enumType: MeetingTypes::class,
         targetEntity: Annulment::class,
         mappedBy: 'target',
     )]
-    protected Annulment $annulledBy;
+    protected ?Annulment $annulledBy = null;
+
+    public function __construct()
+    {
+        $this->subdecisions = new ArrayCollection();
+    }
 
     /**
      * Set the meeting.

module/Decision/src/Model/SubDecision/Installation.php

@@ -58,7 +58,7 @@ class Installation extends FoundationReference
         targetEntity: Discharge::class,
         mappedBy: 'installation',
     )]
-    protected Discharge $discharge;
+    protected ?Discharge $discharge = null;
 
     /**
      * The organmember reference.
@@ -114,7 +114,7 @@ public function getReappointments(): Collection
     /**
      * Get the discharge, if it exists.
      */
-    public function getDischarge(): Discharge
+    public function getDischarge(): ?Discharge
     {
         return $this->discharge;
     }