chore: fix experimental blob repr without content_type (#1337)

Author: GarrettWu

bigframes/dataframe.py

@@ -768,11 +768,11 @@ def _repr_html_(self) -> str:
 
                 def obj_ref_rt_to_html(obj_ref_rt) -> str:
                     obj_ref_rt_json = json.loads(obj_ref_rt)
+                    gcs_metadata = obj_ref_rt_json["objectref"]["details"][
+                        "gcs_metadata"
+                    ]
                     content_type = typing.cast(
-                        str,
-                        obj_ref_rt_json["objectref"]["details"]["gcs_metadata"][
-                            "content_type"
-                        ],
+                        str, gcs_metadata.get("content_type", "")
                     )
                     if content_type.startswith("image"):
                         url = obj_ref_rt_json["access_urls"]["read_url"]