Add default example

Author: thaichat04

src/openapi_parser/specification.py

@@ -168,7 +168,7 @@ class Parameter:
 class Content:
     type: Union[ContentType, LooseContentType]
     schema: Schema
-    example: Optional[Any]
+    example: Optional[Any] = None
     examples: dict[str, Any] = field(default_factory=dict)
     # encoding: dict[str, Encoding]  # TODO