Not iterable error in optional lists constrained by unique items

[nuno-andre]

Initial Checks

• I have searched GitHub for a duplicate issue and I'm sure this is something new
• I have searched Google & StackOverflow for a solution and couldn't find anything
• I have read and followed the docs and still think this is a bug
• I am confident that the issue is with pydantic (not my code, or another library in the ecosystem like FastAPI or mypy)

Description

unique_items validation breaks optional lists when None is passed as a value.

Example Code

from typing import Any, Optional
from pydantic import BaseModel, conlist

class Model(BaseModel):
    prop: Optional[conlist(Any, unique_items=True)]

m = Model(prop=None)

#> pydantic.error_wrappers.ValidationError: 1 validation error for Model
#> prop
#>   'NoneType' object is not iterable (type=type_error)

Python, Pydantic & OS Version

pydantic version: 2.0.0.dev0
install path: C:\repos_me\ppp\pydantic\pydantic
python version: 3.9.8 (tags/v3.9.8:bb3fdcf, Nov  5 2021, 20:48:33) [MSC v.1929 64 bit (AMD64)]
platform: Windows-10-10.0.19044
optional deps. installed: ['email-validator', 'typing-extensions']

Affected Components

• Compatibility between releases
• Data validation/parsing
• Data serialization - .dict() and .json()
• JSON Schema
• Dataclasses
• Model Config
• Field Types - adding or changing a particular data type
• Function validation decorator
• Generic Models
• Other Model behaviour - construct(), pickling, private attributes, ORM mode
• Plugins and integration with other tools - mypy, FastAPI, python-devtools, Hypothesis, VS Code, PyCharm, etc.

[nuno-andre]

Fixed by #4568