Merge bitcoin/bitcoin#23348: rpc, wallet: Do not return "keypoololdest" for blank descriptor wallets

ee03c782ba61993d9e95fa499546cd14cee35445 wallet: Make GetOldestKeyPoolTime return nullopt for blank wallets (Hennadii Stepanov)
3e4f069d23cd2ea5de8fa3c4b1a761ab097ad56f wallet, refactor: Make GetOldestKeyPoolTime return type std::optional (Hennadii Stepanov)

Pull request description:

  The "keypoololdest" field in the `getwalletinfo` RPC response should be used for legacy wallets only.

  Th current implementation (04437ee721e66a7b76bef5ec2f88dd1efcd03b84) assumes that `CWallet::GetOldestKeyPoolTime()` always return `0` for descriptor wallets. This assumption is wrong for _blank_ descriptor wallets, when `m_spk_managers` is empty. As a result:
  ```
  $ src/bitcoin-cli -signet -rpcwallet=211024-d-DPK getwalletinfo
  {
    "walletname": "211024-d-DPK",
    "walletversion": 169900,
    "format": "sqlite",
    "balance": 0.00000000,
    "unconfirmed_balance": 0.00000000,
    "immature_balance": 0.00000000,
    "txcount": 0,
    "keypoololdest": 9223372036854775807,
    "keypoolsize": 0,
    "keypoolsize_hd_internal": 0,
    "paytxfee": 0.00000000,
    "private_keys_enabled": false,
    "avoid_reuse": false,
    "scanning": false,
    "descriptors": true
  }
  ```

  This PR fixes this issue with direct checking of the `WALLET_FLAG_DESCRIPTORS` flag.

ACKs for top commit:
  lsilva01:
    re-ACK ee03c78
  stratospher:
    ACK ee03c78.
  meshcollider:
    Code review ACK ee03c782ba61993d9e95fa499546cd14cee35445

Tree-SHA512: 9852f9f8ed5c08c07507274d7714f039bbfda66da6df65cf98f67bf11a600167d0f7f872680c95775399477f4df9ba9fce80ec0cbe0adb7f2bb33c3bd65b15df

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