message: fix missing abi key mistake
diff: @@ -1120,7 +1120,8 @@ namespace vcpkg::Build
  pre_build_info.passthrough_env_vars = Strings::split(variable_value, ";");
  break;
  case VcpkgTripletVar::PUBLIC_ABI_OVERRIDE :
  - pre_build_info.public_abi_override = variable_value;
  + pre_build_info.public_abi_override =
  + variable_value.empty() ? nullopt : Optional<std::string>{variable_value};
  break;
  }
  }
  