Ensure json-schema generator handles Literal Null types

[bruno-f-cruz]

Change Summary

This PR aims to complete #8944 and #8905 by also handling null types when generating a json-schema from a pydantic model.

For instance, the following model:

from pydantic import BaseModel
from typing import Literal


class Foo(BaseModel):
    bar: Literal["Bar"] = 'Bar'
    baz: Literal[None] = None
    foo: str = 'Foo'

leads to:

{'properties': {'bar': {'const': 'Bar', 'default': 'Bar', 'enum': ['Bar'], 'title': 'Bar', 'type': 'string'}, 'baz': {'const': None, 'default': None, 'enum': [None], 'title': 'Baz', 'type': 'null'}, 'foo': {'default': 'Foo', 'title': 'Foo', 'type': 'string'}}, 'title': 'Foo', 'type': 'object'}

Related issue number

fix #8905

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @adriangb

[codspeed-hq]

CodSpeed Performance Report

Merging #9135 will not alter performance

_{Comparing bruno-f-cruz:add-literal-null-to-json-schema (65f4461) with main (8128821)}

Summary

✅ 13 untouched benchmarks

[bruno-f-cruz]

please review @sydney-runkle Not sure why those 3 tests are failing, can you take a look? thanks!

[sydney-runkle]

@bruno-f-cruz,

Looked like flaky CI. All is well!

[sydney-runkle]

Looks great, thanks!

Waiting for final confirmation from @dmontagu before we merge.