Set default value (or empty list/dict) no longer unset the field

[bagerard]

This fixes an awkward historical behavior of mongoengine. Fields that have falsy, non-numeric default values would be unset upon saving.

Typically:

  class UserProfile(Document):
      display_name = StringField(default="")
      interests = ListField(StringField())


  UserProfile.drop_collection()

  profile = UserProfile(
      display_name="Alice",
      interests=["reading", "cycling"],
  ).save()

  profile.display_name = ""
  profile.interests = []
  profile.save()

  print(UserProfile.objects.as_pymongo())

Historically would remove the fields from the documents
[{"_id": ObjectId("...")}]

And will now produce:
[{'_id': ObjectId('...'), 'display_name': '', 'interests': [ ] } ]

Fixes #267
Fixes #761
Relates to #2342
closes 2517