add interface

src/Illuminate/Http/Resources/ConditionallyLoadsAttributes.php

@@ -29,9 +29,10 @@ protected function filter($data)
                 return $this->merge($data, $index, $this->filter($value->data));
             }
 
-            if ($value instanceof MissingValue ||
+            if (($value instanceof PotentiallyMissing && $value->isMissing()) ||
                 ($value instanceof self &&
-                $value->resource instanceof MissingValue)) {
+                $value->resource instanceof PotentiallyMissing &&
+                $value->isMissing())) {
                 unset($data[$key]);
 
                 $index--;

src/Illuminate/Http/Resources/MissingValue.php

@@ -2,7 +2,15 @@
 
 namespace Illuminate\Http\Resources;
 
-class MissingValue
+class MissingValue implements PotentiallyMissing
 {
-    //
+    /**
+     * Determine if the object should be considered "missing".
+     *
+     * @return bool
+     */
+    public function isMissing()
+    {
+        return true;
+    }
 }

src/Illuminate/Http/Resources/PotentiallyMissing.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace Illuminate\Http\Resources;
+
+interface PotentiallyMissing
+{
+    /**
+     * Determine if the object should be considered "missing".
+     *
+     * @return bool
+     */
+    public function isMissing();
+}