chore: fix lints in UnionOf

Author: stainless-app[bot]

src/Core/Conversion/UnionOf.php

@@ -65,7 +65,7 @@ public function coerce(mixed $value, CoerceState $state): mixed
 
     public function dump(mixed $value, DumpState $state): mixed
     {
-        if (!is_null($target = $this->resolveVariant(value: $value))) {
+        if (null !== ($target = $this->resolveVariant(value: $value))) {
             return Conversion::dump($target, value: $value, state: $state);
         }
 
@@ -85,8 +85,15 @@ private function resolveVariant(
             return $value::class;
         }
 
-        if (!is_null($this->discriminator) && is_array($value) && array_key_exists($this->discriminator, array: $value)) {
+        if (
+            null !== $this->discriminator
+            && is_array($value)
+            && array_key_exists($this->discriminator, array: $value)
+        ) {
             $discriminator = $value[$this->discriminator];
+            if (!is_string($discriminator)) {
+                return null;
+            }
 
             return $this->variants[$discriminator] ?? null;
         }