tflite model graph output name defaults to an empty list when using from_tflite

[PaulWassermann]

Describe the bug

When converting a tflite model to an onnx ModelProto using the newly-available function tf2onnx.convert.from_tflite, if the output_names argument is not specified, it defaults to an empty list instead of None.

However, in the tf2onnx.tflite_utils.graphs_from_tflite function, if the output_names argument is not None, the output names retrieved automatically by the tf2onnx.tflite_utils.parse_tflite_graph function are overwritten, resulting in a ModelProto with no output.

System information

• OS Platform and Distribution: Windows 10
• Tensorflow version: 2.8.0
• tf2onnx version: 1.11.1
• Python version: 3.8.13

To Reproduce

Use the tf2onnx.convert.from_tflite function to convert a tflite model without specifying the output_names argument. This results in the returned ModelProto.graph to have its output attribute set to [].

Additional context
A simple fix would be to delete those 2 lines

if output_names is None:
    output_names = []

at the top of the tf2onnx.convert.from_tflite function. It would also be more consistent with the CLI equivalent:

python -m tf2onnx.convert --tflite <path_to_tflite> --output <output_path>

that results in the output_names argument to default to None (which means no bug when using the CLI).