I hope I didn’t mess anything around. This is my code now:
# create Hugging Face Model Class
huggingface_model = HuggingFaceModel(
   model_data=model_uri, # configuration for loading model from Hub
   role=role, # iam role with permissions to create an Endpoint
   transformers_version="4.6", # transformers version used
   pytorch_version="1.7", # pytorch version used
   py_version='py36', # python version used
)

# create Transformer to run our batch job
batch_job = huggingface_model.transformer(
    instance_count=1,
    instance_type='ml.g4dn.xlarge',
    output_path=output_s3_path, # we are using the same s3 path to save the output with the input
    strategy='SingleRecord',accept="application/json")

# starts batch transform job and uses s3 data as input
batch_job.transform(
    data=s3_file_uri,
    content_type='application/json',    
    split_type='Line',join_source='Input')

And this is my output:
ClientError: An error occurred (ValidationException) when calling the CreateTransformJob operation: SplitType ("Line") in TransformInput has to be the same as AssembleWith ("null") in TransformOutput when JoinSource is "Input".