price data failed to be merged into dataframe when there's ticker with no data

[zhenyulin]

Describe the bug

yahooquery/ticker.py in _historical_data_to_dataframe(self, data, params, adj_timezone)
   1267         d = {}
   1268         for symbol in self._symbols:
-> 1269             if "timestamp" in data[symbol]:
   1270                 d[symbol] = _history_dataframe(data, symbol, params, adj_timezone)
   1271             else:

just need a simple null check here

[danilogalisteu]

See solution in #97 and pull request #102