Type definition for Field.__set__ does not expect None to be passed even if the field is nullable

[zwx00]

Describe the bug
Trying to assign None to a field that has null=True gives a type error.

To Reproduce

import tortoise
from tortoise import fields

class Test(tortoise.Model):
    id = fields.IntField(pk=True)
    name = fields.CharField(max_length=255, null=True)

a = Test(name="test")
a.name = None

Resulting type checks:

git:(main) ✗ uv run mypy .      
test.py:12: error: Incompatible types in assignment (expression has type "None", variable has type "str")  [assignment]
Found 1 error in 1 file (checked 2 source files)
git:(main) ✗ uv run pyright .
/Users/zw/Development/tortoise-typebug/test.py
  /Users/zw/Development/tortoise-typebug/test.py:12:3 - error: Cannot assign to attribute "name" for class "Test"
    Argument of type "None" cannot be assigned to parameter "value" of type "str" in function "__set__"
      "None" is not assignable to "str" (reportAttributeAccessIssue)
1 error, 0 warnings, 0 informations 

Expected behavior
Should be allowed to set a field to None. It's a legal value and the type definition should reflect this.
Additional context
Happy to help out with a PR if needed. Would appreciate some pointers.

https://github.com/tortoise/tortoise-orm/blob/develop/tortoise/fields/base.py#L165

Here's the problematic code. This is causing an issue because CharField is defined as follows:

class CharField(Field[str]):
    ...

I'm also including my current workaround which works but requires lot of copypaste boilerplate.

class RequiredUUIDField(fields.UUIDField):
    def __set__(self, instance: "Model", value: uuid.UUID) -> None:
        super().__set__(instance, value)


class NullableUUIDField(fields.UUIDField):
    def __set__(self, instance: "Model", value: Optional[uuid.UUID]) -> None:
        super().__set__(instance, value)  # type: ignore


@overload
def UUIDField(null: Literal[False] = False, **kwargs) -> RequiredUUIDField: ...


@overload
def UUIDField(null: Literal[True], **kwargs) -> NullableUUIDField: ...


def UUIDField(null: bool = False, **kwargs):
    if null:
        return NullableUUIDField(null=True, **kwargs)
    return RequiredUUIDField(**kwargs)