Add default none for Field correctly

[brian-goo]

None should be added if nothing was specified for default of Field if I understood correctly.
This PR is to fix #113 as well.

Before:

class Foo(BaseModel):
    a: Optional[int] = Field() # here
    b: Optional[int] = Field(...)
    c: Optional[int] = Field(None, description="spam")
    d: Optional[int] = Field(description="spam") # here
    e: Optional[int] = Field(default=..., description="spam")
    f: Optional[int] = Field(default=None, description="spam")

After:

class Foo(BaseModel):
    a: Optional[int] = Field(None) # here
    b: Optional[int] = Field(...)
    c: Optional[int] = Field(None, description="spam")
    d: Optional[int] = Field(None, description="spam") # here
    e: Optional[int] = Field(default=..., description="spam")
    f: Optional[int] = Field(default=None, description="spam")

[Kludex]

This is wrong. You shouldn't have both default and default_factory being set.

[Kludex]

• I've replaced this by Don't add default None when default is ... #127

I've added you as a co-author @brian-goo FYI. 🙏