message: wallet: Make GetOldestKeyPoolTime return nullopt for blank wallets
  This change suppress the "keypoololdest" field in the getwalletinfo RPC
  response for blank descriptor wallets.
diff: @@ -2215,6 +2215,10 @@ bool CWallet::GetNewChangeDestination(const OutputType type, CTxDestination& des
  std::optional<int64_t> CWallet::GetOldestKeyPoolTime() const
  {
  LOCK(cs_wallet);
  + if (m_spk_managers.empty()) {
  + return std::nullopt;
  + }
  +
  std::optional<int64_t> oldest_key{std::numeric_limits<int64_t>::max()};
  for (const auto& spk_man_pair : m_spk_managers) {
  oldest_key = std::min(oldest_key, spk_man_pair.second->GetOldestKeyPoolTime());
  