openaiphp phpunit failure fix (#375)

Author: smaddock

src/Instrumentation/OpenAIPHP/src/OpenAIPHPInstrumentation.php

@@ -193,7 +193,7 @@ private static function hookApi(CachedInstrumentation $instrumentation, $class,
 
     private static function recordUsage(SpanInterface $span, object $response, ContextInterface $context)
     {
-        if (!property_exists($response, 'usage') || !method_exists($response->usage, 'toArray')) {
+        if (!property_exists($response, 'usage') || !isset($response->usage) || !method_exists($response->usage, 'toArray')) {
             return;
         }