Deserialization fails for Optional[Dict[...]]

[alexmikoto]

Trying to deserialize a nullable Dict[T, U] results in get_dict_parser being called anyway and raising an AttributeError due to the target dict being None.

Testing with Python 3.7 and dataclass_factory 2.4.1 the following code will raise:

from typing import Optional, Dict, Any
from dataclass_factory import Factory

@dataclass
class Test:
    x: Optional[Dict[str, Any]]

f = Factory()
f.load({"x": None}, Test)

The root issue seems to be that Optional is not being detected correctly as it is indistinguishable from Union[T, NoneType], which ends up in get_dict_parser raising due to the order of the type arguments.