Issue number: #5887

Summary

Changes

This PR fixes a bug where OpenAPI validation was not validating response serialization when the response body was a falsy value (e.g., False, 0, empty string, etc.), making the the response body validation is more strict.
It also removes Pydantic V1 compatibility.

User experience

Before:

class Todo(BaseModel):
    title: str
    status: Literal["Pending", "Done"]

@app.get("/none")
def return_none() -> Todo:
    return None # Returns 200

@app.get("/empty_list")
def return_empty_list() -> Todo:
    return list() # Returns 200

After:

class Todo(BaseModel):
    title: str
    status: Literal["Pending", "Done"]

@app.get("/none")
def return_none() -> Optional[Todo]:
    return None # Returns 200 -> Optional 

@app.get("/empty_list")
def return_empty_list() -> Todo:
    return list() # Returns 422 - validation error

Checklist

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• Meet tenets criteria
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• PR title follows conventional commit semantics

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