Fix export Conv operator with no bias

Context

When exporting conv operators with no bias, the corresponding input is stored as a None node. This leads to errors when exporting the network.

Modified files

• aidge_onnx/node_export/aidge_converters/conv.py;
• aidge_onnx/node_export/aidge_converters/padded_conv.py;
• aidge_onnx/node_export/aidge_converters/padded_maxpooling.py;
• aidge_onnx/onnx_export.py;

Detailed major modifications

When an input is None only a warning is printed in verbose mode instead of the Exception. Otherwise, it consists of bug fix.

https://github.com/astral-sh/ruff formatter is used on the modified files.