[bug] JSON validation converts null values to True:bool

[1-Archit-1]

Describe the Bug
On validation using guard null values returned by the llm for specified fields is converted to True.

To Reproduce

import openai
from guardrails import Guard
from pydantic import BaseModel, Field
from typing import Optional

class Car(BaseModel):
    name: str = Field(description="name of the car")
    color: Optional[str] = Field(description="This field is nullable")

guard = Guard.from_pydantic(output_class=Car)

raw_llm_output, validated_output, *others =  response = guard(
    llm_api=openai.chat.completions.create,
    prompt="""Generate a car object. Only add fields described below. Do not add any new fields
    ${gr.complete_json_suffix}"""
)

print(validated_output)

Expected behavior
The validated output should be {'car': 'Honda, 'color':None}
Instead we get {'car': 'Honda, 'color':True}

Library version:
Version (e.g. 0.4.0)

[CalebCourier]

Fixed in 0.4.1

See #604
and #479