Enhancement: allow route handlers to return None if return_dto is set without configuring partial

[LonelyVikingMichael]

Summary

Refer to the example below.

A route handler annotated to return Data | None, along with return_dto=DataDTO being set will raise AttributeError if None is returned - if the DTO config partial attribute is False. The same goes for returning an empty dict, except this will raise a KeyError instead.

A workaround is to define a duplicate DTO with partial set True.

Basic Example

class FooModel(Struct):
    bar: str
    
class FooDTO(MsgspecDTO[FooModel):
    config = DTOConfig(partial=False)  # default but okay

@get("/foo", return_dto=FooDTO)
def get_foo() -> FooModel | None:
    # some condition evaluates `foo` to `None`
    return None  # raises AttributeError: `NoneType` has no attribute 'bar'    

Drawbacks and Impact

This enhancement might cause issues for users following this pattern and actively catching attribute or key errors. People are crazy.

Unresolved questions

No response

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