message: wallet: Replace boost::none with nullopt
diff: @@ -215,7 +215,7 @@ static void SetFeeEstimateMode(const CWallet* pwallet, CCoinControl& cc, const U
  cc.m_feerate = CFeeRate(fee_rate);
  // default RBF to true for explicit fee rate modes
  - if (cc.m_signal_bip125_rbf == boost::none) cc.m_signal_bip125_rbf = true;
  + if (cc.m_signal_bip125_rbf == nullopt) cc.m_signal_bip125_rbf = true;
  } else if (!estimate_param.isNull()) {
  cc.m_confirm_target = ParseConfirmTarget(estimate_param, pwallet->chain().estimateMaxBlocks());
  }
  