Update a field in Mongoose if the new value is not null?

having a Schema like this:

const FarmSchema = new Schema({
  name: {
    type: String,
    required: true
  },
  extension: {
    extension: {
      type: Number,
      min: 1,
      max: 1000,
      required: true
    },
    measure: {
      type: String,
      enum: ['Hectareas', 'Manzanas'],
      required: true
    }
  },
  animals: [mongoose.model('Animal').schema],
  outs: {
    type: Array,
    required: true
  },
  outs: {
    type: Array,
    required: true
  },
  plans: {
    type: Array,
    required: true
  }
}, {
  timestamps: true,
  useNestedStrict: true
});

And in the controller to update Farms this:

exports.update = (req, res) => {
  User.update(
    {
      username: req.params.username,
      'farms._id' : req.params.idFarm
    },
    {
      $set: { // fix me 
        'farms.$.name': req.body.name,
        'farms.$.extension': req.body.extension
      }
    }
  )
    .then(farm => {
      res.send(farm);
    })
    .catch(err => {
      res.status(500).send({
        message:
          err.message || "Some error occurred while retrieving all farms."
      });
    });
};

What I want is that if the user only wants to modify the 'name' field, we send a JSON like this to the server and only update the name field without changing the 'extension' field:

{
    "name": "Finca 99"
}

The problem is that if I do this, the 'extension' field becomes null or vice versa.