return None when next bar is not available

_load method of the feed should returned False when price
data is not yet available. This caused cerebro to stop.
It should return None in this case to indicate timeout
getting the data.

Signed-off-by: Ed Bartosh <bartosh@gmail.com>

backtrader/feeds/ccxt.py

@@ -150,7 +150,7 @@ def _load_ticks(self):
         try:
             trade = self._data.popleft()
         except IndexError:
-            return False # no data in the queue
+            return None # no data in the queue
 
         trade_time, price, size = trade