fix: Check for NoneType extension in serialization

[NaqGuug]

Sometimes partial_result can be None when serialising extension. If so, partial_result.items() will raise an error.

This PR just adds a small check for possible NoneType.

[NaqGuug]

To be more precise, this happens when passing a scim model with empty extensions to FastAPI endpoint with a specific response model.

Example

app = FastAPI()

@app.get(
  "/endpoint/",
  response_model=User[EnterpriseUser]
)
async def get_model() -> User[EnterpriseUser]:
  # EnterpriseUser is None, will raise an error
  # when FastAPI tries to serialise the object.
  return User[EnterpriseUser]() 

[azmeuk]

This is released in 0.6.5