fix bug for 0 transaction wallet

0 transaction wallet would return an error when getting transaction list

plugins/exchange_rate.py

@@ -345,7 +345,10 @@ class Plugin(BasePlugin):
         if self.config.get('history_rates', 'unchecked') == "checked":
             tx_list = self.tx_list
            
+            try:
                 mintimestr = datetime.datetime.fromtimestamp(int(min(tx_list.items(), key=lambda x: x[1]['timestamp'])[1]['timestamp'])).strftime('%Y-%m-%d')
+            except ValueError:
+                return
             maxtimestr = datetime.datetime.now().strftime('%Y-%m-%d')
             try:
                 resp_hist = self.exchanger.get_json('api.coindesk.com', "/v1/bpi/historical/close.json?start=" + mintimestr + "&end=" + maxtimestr)