You may have misremembered it, there's no such "pre-wallet era".
In the very old versions including the first release, Bitcoin Core will create a default wallet for you.

The difference is:
You'll have to explicitly create a wallet in the new versions.
It wasn't possible to create multiple wallets back then aside from manually swapping wallet.dat files.

For mining in the old version: When you tick "Generate Coins", it'll auto-select one of your wallet's address in the background.
And of course, it will not show coins if you didn't mined a block since it's solo mining (either 50BTC or zero) or no one sent you bitcoins.