reduce serialization of null models

Models/NullCategory.php

@@ -35,4 +35,12 @@ final class NullCategory extends Category
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }

Models/NullCause.php

@@ -35,4 +35,12 @@ final class NullCause extends Cause
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }

Models/NullDepartment.php

@@ -35,4 +35,12 @@ final class NullDepartment extends Department
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }

Models/NullProcess.php

@@ -35,4 +35,12 @@ final class NullProcess extends Process
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }

Models/NullProject.php

@@ -35,4 +35,12 @@ final class NullProject extends Project
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }

Models/NullRisk.php

@@ -35,4 +35,12 @@ final class NullRisk extends Risk
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }

Models/NullRiskObject.php

@@ -35,4 +35,12 @@ final class NullRiskObject extends RiskObject
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }

Models/NullSolution.php

@@ -35,4 +35,12 @@ final class NullSolution extends Solution
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }