null check

[rohanricky]

Summary

null check is missing in $__set method which is causing TypeError: Cannot set properties of null (setting 'isNew')

Repro script

var mongoose = require('mongoose')
console.log(mongoose.version)
let Schema = mongoose.Schema
var eventSchema = new Schema({
  name: { type: String },
  field3: { type: Schema.Types.Mixed },
  __stateBeforeSuspension: {
    field1: { type: String },
    field2: { type: String },
    jsonField: {
        name: { type: String },
        ex: { type: String }
    }
  }
})

eventSchema.add({
  test: { 
    val1: { type: String },
    val2: { type: String }
   }
})

eventSchema.add({
  subSchema4: [{ field1: { type: String } }, { field2: { type: String } }]
})

eventObj.save(function (err, eventObj) {
  var newObject = eventObj.toObject()
  newObject.subSchema4[0] = null
  eventObj.set(newObject)

  eventObj.save(function (err, eventObj1) {

    var newObject2 = eventObj1.toObject()
    newObject2.__stateBeforeSuspension.jsonField.name = 'test4'
    eventObj1.set(newObject2)
  })
})

[vkarpov15]

LGTM. Looks like this is a duplicate of #13859, which was fixed by #13883. I'll add a test case from #13883 after merging