Inconsistent handling of fields without a value if dataclass or not.

[omry]

Fields annotated as dataclass without a value auto-expends but primitive fields are turning into MISSING.
In the example below, User and age are treated differently.
I would expect both to be MISSING ('???').
Fixing it is a breaking change, need to evaluate it carefully.

from omegaconf import OmegaConf
from dataclasses import dataclass, field
from typing import Dict

@dataclass
class User:
    age: int
    name: str = "foo"

@dataclass
class Config:
    user: User

cfg : Config = OmegaConf.structured(Config)
print(cfg)


$ python 2.py 
{'user': {'age': '???', 'name': 'foo'}}

[omry]

Done, duplicate of #390.