Guard against null data in Post._obtain_metadata (#2705)

When Instagram returns {"data": null} for a shortcode metadata query (e.g. under
rate limiting, or for an unavailable post), response["data"]["xdt_shortcode_media"]
raised a raw TypeError before the existing `if pic_json is None` check could run.
Guard the intermediate "data" so the clean BadResponseException is raised instead,
which the download loop already handles.

instaloader/structures.py

@@ -319,9 +319,11 @@ class Post:
 
     def _obtain_metadata(self):
         if not self._full_metadata_dict:
-            pic_json = self._context.doc_id_graphql_query(
+            response = self._context.doc_id_graphql_query(
                 "8845758582119845", {"shortcode": self.shortcode}
-            )["data"]["xdt_shortcode_media"]
+            )
+            data = (response or {}).get("data")
+            pic_json = data.get("xdt_shortcode_media") if data else None
             if pic_json is None:
                 raise BadResponseException("Fetching Post metadata failed.")
             try: