schema_json() fails for optional namedtuple fields with default value

[unti1x]

Checks

• I added a descriptive title to this issue
• I have searched (google, github) for similar issues and couldn't find anything
• I have read and followed the docs and still think this is a bug

Bug

When trying to generate a schema for a model with an optional NamedTuple field which have a default value, schema_json() fails when trying to unpack parameters for the tuple:

Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "pydantic/main.py", line 715, in pydantic.main.BaseModel.schema_json
  File "pydantic/main.py", line 704, in pydantic.main.BaseModel.schema
  File "pydantic/schema.py", line 167, in pydantic.schema.model_schema
  File "pydantic/schema.py", line 548, in pydantic.schema.model_process_schema
  File "pydantic/schema.py", line 589, in pydantic.schema.model_type_schema
  File "pydantic/schema.py", line 234, in pydantic.schema.field_schema
  File "pydantic/schema.py", line 202, in pydantic.schema.get_field_info_schema
  File "pydantic/schema.py", line 892, in genexpr
TypeError: <lambda>() missing 1 required positional argument: 'y'

 def encode_default(dft: Any) -> Any:
     if isinstance(dft, (int, float, str)):
         return dft
     elif sequence_like(dft):
         t = dft.__class__
>        return t(encode_default(v) for v in dft)
   ...

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.8.1
            pydantic compiled: True
                 install path: /venv/lib/python3.9/site-packages/pydantic
               python version: 3.9.0+ (default, Oct 20 2020, 08:43:38)  [GCC 9.3.0]
                     platform: Linux-5.4.0-72-generic-x86_64-with-glibc2.31
     optional deps. installed: ['dotenv', 'typing-extensions']

from typing import List, Optional, NamedTuple
from pydantic import BaseModel


class Coordinates(NamedTuple):
    x: float
    y: float


class LocationBase(BaseModel):
    name: str
    coords: Optional[Coordinates] = Coordinates(0, 0)
    zoom: Optional[int] = 10


LocationBase.schema_json()

[PrettyWood]

Hi @unti1x
Thanks for reporting! I made a quick fix.