Fix ReflectionClass::getConstants() stub

If `zval_update_constant_ex()` fails, an exception has already been
thrown, so we clarify that in the implementation as well.

Closes GH-6557.

ext/reflection/php_reflection.stub.php

@@ -259,7 +259,7 @@ class ReflectionClass implements Reflector
     /** @return bool */
     public function hasConstant(string $name) {}
 
-    /** @return array|null */
+    /** @return array */
     public function getConstants(?int $filter = null) {}
 
     /** @return ReflectionClassConstant[] */