Error when fetch the history data with minute timeframe.

Hi,

I encounter this error when I upgrade to version alpaca-backtrader-api-0.14.1. It can be reproduced using the code of strategy_readme_sample.py . The only change is backtest and the timeframe of minute bars.

File "/Users/xxx/lib/python3.8/site-packages/alpaca_backtrader_api/alpacastore.py", line 397, in _t_candles
cdl = self.get_aggs_from_alpaca(dataname,
File "/Users/xxx/lib/python3.8/site-packages/alpaca_backtrader_api/alpacastore.py", line 586, in get_aggs_from_alpaca
response = _iterate_api_calls()
File "/Users/xxx/lib/python3.8/site-packages/alpaca_backtrader_api/alpacastore.py", line 522, in _iterate_api_calls
r = self.oapi.get_bars(dataname,
File "/Users/xxx/lib/python3.8/site-packages/alpaca_trade_api/rest.py", line 651, in get_bars
bars = list(self.get_bars_iter(symbol,
File "/Users/xxx/lib/python3.8/site-packages/alpaca_trade_api/rest.py", line 637, in get_bars_iter
for bar in bars:
File "/Users/xxx/lib/python3.8/site-packages/alpaca_trade_api/rest.py", line 564, in _data_get_v2
for item in items:
TypeError: 'NoneType' object is not iterable

The code:

```
import alpaca_backtrader_api
import backtrader as bt
from datetime import datetime

ALPACA_API_KEY = ""
ALPACA_SECRET_KEY = ""

IS_BACKTEST = True
IS_LIVE = False
symbol = "AAPL"

class SmaCross(bt.SignalStrategy):
def __init__(self):
sma1, sma2 = bt.ind.SMA(period=10), bt.ind.SMA(period=30)
crossover = bt.ind.CrossOver(sma1, sma2)
self.signal_add(bt.SIGNAL_LONG, crossover)

if __name__ == '__main__':
cerebro = bt.Cerebro()
cerebro.addstrategy(SmaCross)

store = alpaca_backtrader_api.AlpacaStore(
key_id=ALPACA_API_KEY,
secret_key=ALPACA_SECRET_KEY,
paper=not IS_LIVE,
)

DataFactory = store.getdata # or use alpaca_backtrader_api.AlpacaData
if IS_BACKTEST:
data0 = DataFactory(dataname=symbol, historical=True,
fromdate=datetime(2021, 8, 12),
timeframe=bt.TimeFrame.Minutes,
data_feed='iex')
else:
data0 = DataFactory(dataname=symbol,
historical=False,
timeframe=bt.TimeFrame.Days,
data_feed='iex')
# or just alpaca_backtrader_api.AlpacaBroker()
broker = store.getbroker()
cerebro.setbroker(broker)
cerebro.adddata(data0)

print('Starting Portfolio Value: {}'.format(cerebro.broker.getvalue()))
cerebro.run()
print('Final Portfolio Value: {}'.format(cerebro.broker.getvalue()))
cerebro.plot()
```

I found that the earliest_sample in _iterate_api_calls() is some time in the day of fromdate(start) like 2021-08-12 04:03:00 which is never earlier than the start date time 2021-08-12 00:00:00. So the got_all is till false when the data fetch is done. It works for the days timeframe but failed on minutes timeframe. I am not sure if that is the root cause, and just post my finding.