dont use pd.read_json() because it can't handle NaN

pandas choosing not to be pragmatic and not support them,
so I choose to use the standard library json module which
does

pandas-dev/pandas#12213

Author: nzjrs

imgstore/stores.py

@@ -434,7 +434,9 @@ def get_extra_data(self):
             for ext in ('.extra.json', '.extra_data.json'):
                 path = chunk_path + ext
                 if os.path.exists(path):
-                    dfs.append(pd.read_json(path, orient='record'))
+                    with open(path, 'rt') as f:
+                        records = json.load(f)
+                    dfs.append(pd.DataFrame(records))
         return pd.concat(dfs, axis=0, ignore_index=True)
 
     def add_extra_data(self, **data):