I have a bunch of old wallet.dat files on my computer who are most likely empty. Just to be safe, I am trying to find a way to check their balance without having to re-sync the entire blockchain all the time.

I have been successful in using the command dumpwallet, get the extended private masterkey and import it into Electrum. This was a non-encrypted wallet.

I have two more wallets that are encrypted and I know the password. I have used the command walletpassphrase "PASSWORD" 60 to unlock them for one minute and I immediately run the command dumpwallet. I was able to obtain the dump, but it does not contain any extended private masterkey. It's just a list of BTC addresses. Why is that so and how can I get the extended private masterkey for these wallet files?