Use correct default type for str (#8623)

* [Python] Use correct type for str with None

Otherwise mypy will correctly flag code like this

def __init__(self):
  self.fooBar = None  # type: Optional[str]

error: Incompatible types in assignment (expression has type "None", variable has type "str")

* [Python] Make list type optional as they can contain None

tests/MyGame/Example/Stat.py

@@ -84,7 +84,7 @@ class StatT(object):
 
     # StatT
     def __init__(self):
-        self.id = None  # type: str
+        self.id = None  # type: Optional[str]
         self.val = 0  # type: int
         self.count = 0  # type: int