[Serializer] Fix cache in MetadataAwareNameConverter

`isset` is used to test existence of values that is
`null` by default, which result to always bypass the cache
and force to do the calculate all the time.

This is a critical perf improvement in prod mode for an api.

Ref #35085

NameConverter/MetadataAwareNameConverter.php

@@ -47,7 +47,7 @@ public function normalize($propertyName, string $class = null, string $format =
             return $this->normalizeFallback($propertyName, $class, $format, $context);
         }
 
-        if (!isset(self::$normalizeCache[$class][$propertyName])) {
+        if (!\array_key_exists($class, self::$normalizeCache) || !\array_key_exists($propertyName, self::$normalizeCache[$class])) {
             self::$normalizeCache[$class][$propertyName] = $this->getCacheValueForNormalization($propertyName, $class);
         }
 
@@ -64,7 +64,7 @@ public function denormalize($propertyName, string $class = null, string $format
         }
 
         $cacheKey = $this->getCacheKey($class, $context);
-        if (!isset(self::$denormalizeCache[$cacheKey][$propertyName])) {
+        if (!\array_key_exists($cacheKey, self::$denormalizeCache) || !\array_key_exists($propertyName, self::$denormalizeCache[$cacheKey])) {
             self::$denormalizeCache[$cacheKey][$propertyName] = $this->getCacheValueForDenormalization($propertyName, $class, $context);
         }
 
@@ -78,7 +78,7 @@ private function getCacheValueForNormalization(string $propertyName, string $cla
         }
 
         $attributesMetadata = $this->metadataFactory->getMetadataFor($class)->getAttributesMetadata();
-        if (!isset($attributesMetadata[$propertyName])) {
+        if (!\array_key_exists($propertyName, $attributesMetadata)) {
             return null;
         }
 
@@ -93,7 +93,7 @@ private function normalizeFallback(string $propertyName, string $class = null, s
     private function getCacheValueForDenormalization(string $propertyName, string $class, array $context): ?string
     {
         $cacheKey = $this->getCacheKey($class, $context);
-        if (!isset(self::$attributesMetadataCache[$cacheKey])) {
+        if (!\array_key_exists($cacheKey, self::$attributesMetadataCache)) {
             self::$attributesMetadataCache[$cacheKey] = $this->getCacheValueForAttributesMetadata($class, $context);
         }