fix infinity values in LetsExchange fixed rate (#1849)

Co-authored-by: Omar Hatem <omarh.ismail1@gmail.com>

lib/exchange/provider/letsexchange_exchange_provider.dart

@@ -97,6 +97,8 @@ class LetsExchangeExchangeProvider extends ExchangeProvider {
 
       final amountToGet = double.tryParse(responseJSON['amount'] as String) ?? 0.0;
 
+      if (amountToGet == 0.0) return 0.0;
+
       return isFixedRateMode ? amount / amountToGet : amountToGet / amount;
     } catch (e) {
       log(e.toString());