Exchange rates equal to 1 instead of NA for missing data at ECB

[albangp]

Hello,

When using the following instruction, all the retrieved exchange rates are equal to 1.
historical_exchange_rates(from="RUB", to="EUR", start_date="2000-01-01", end_date="2000-01-10")

As this package is based on ECB, exchange rates with rubles are available starting only on 2005-04-01.
I think missing data (which actually existed, see here the EUR/RUB in 2000) should be returned as NA instead of 1.

Thank you!

Alban