Fix crash when operation defaults are missing (#1658)

Given that `??` has lower precedence than `+` ([docs](http://php.net/manual/en/language.operators.precedence.php)), this snippet that was intended to fall back to an empty array if defaults are missing instead gave a guaranteed crash as it first tried to add an array and `null` together before coalescing the result.

src/Bridge/Symfony/Routing/ApiLoader.php

@@ -119,7 +119,7 @@ public function load($data, $type = null): RouteCollection
                             'collection' => $operation['collection'],
                             'operationId' => $operationId,
                         ],
-                    ] + $operation['defaults'] ?? [],
+                    ] + ($operation['defaults'] ?? []),
                     $operation['requirements'] ?? [],
                     $operation['options'] ?? [],
                     $operation['host'] ?? '',