Field keypoololdest returned from getwalletinfo should not be required

[craigraw]

As per https://github.com/bitcoin/bitcoin/blob/e334f7a54592ba9f05e4a5578dd933a7a31c3444/src/wallet/rpc/wallet.cpp#L91 the field keypoololdest is optional and not returned if no keys have been imported into the wallet yet. However, BWT (via rust-bitcoincore-rpc) requires it, causing BWT to fail to start up in this situation.

Note that currently Bitcoin Core does actually return keypoololdest when the wallet is first created, even if no keys are added. However, if the wallet is unloaded and then reloaded, the field is missing. The only remedy at this point is to shut down Bitcoin Core, delete the wallet entirely and start again, making sure to add keys before unloading the wallet.