exchange_rate: "BTC" to "BTC" rate is not guaranteed to be present

Which ccy rates are available depends on the configured
exchange (config key use_exchange) and the configured currency (config
key currency). Only for some exchanges, the fx.ccy-BTC fx rate is
available (depends on the ExchangeBase.get_rates implementation).

As they say, for hodlers 1 BTC = 1 BTC.

electrum/exchange_rate.py

@@ -176,6 +176,8 @@ class ExchangeBase(Logger):
 
     def get_cached_spot_quote(self, ccy: str) -> Decimal:
         """Returns the cached exchange rate as a Decimal"""
+        if ccy == 'BTC':
+            return Decimal(1)
         rate = self._quotes.get(ccy)
         if rate is None:
             return Decimal('NaN')