Improve type annotations where objects can be null

fixes #34

Author: cebe

src/spec/Link.php

@@ -19,7 +19,7 @@
  * @property array $parameters
  * @property mixed $requestBody
  * @property string $description
- * @property Server $server
+ * @property Server|null $server
  *
  */
 class Link extends SpecBaseObject

src/spec/Operation.php

@@ -20,8 +20,8 @@
  * @property ExternalDocumentation|null $externalDocs
  * @property string $operationId
  * @property Parameter[]|Reference[] $parameters
- * @property RequestBody|Reference $requestBody
- * @property Responses $responses
+ * @property RequestBody|Reference|null $requestBody
+ * @property Responses|null $responses
  * @property Callback[]|Reference[] $callbacks
  * @property bool $deprecated
  * @property SecurityRequirement[] $security