Verify sell-rate got a value - otherwise downstream code does not work.

Using PricingException here will cease operation for this pair for this
iteration - postponing handling to the next iteration - where hopefully
a price is again present.

freqtrade/freqtradebot.py

@@ -676,6 +676,8 @@ class FreqtradeBot:
                 raise PricingError from e
         else:
             rate = self.exchange.fetch_ticker(pair)[ask_strategy['price_side']]
+        if rate is None:
+            raise PricingError(f"Sell-Rate for {pair} was empty.")
         self._sell_rate_cache[pair] = rate
         return rate