SegFault during shape inference if the schema not be set inference function

[OYCN]

Bug Report

Describe the bug

SegFault during shape inference if the schema not be set inference function

Reproduction instructions

cpp (Optional):

ONNX_OPERATOR_SCHEMA(CustomOp);

python:

import onnx
from onnx import defs

input = """
            <
                ir_version: 7,
                opset_import: [
                    "" : 1
                ]
            >
            agraph (float[N, 128] X, int32 Y) => (float[N] Z)
            {
                Z = CustomOp(X, Y)
            }
           """
model = onnx.parser.parse_model(input)

op_schema = defs.OpSchema(
    'CustomOp',
    '',
    1,
    inputs=[
    ],
    outputs=[
    ],
)

# Uncomment if not register in cpp
# onnx.defs.register_schema(op_schema)
onnx.shape_inference.infer_shapes(model, True)

Expected behavior

Raise a python exception rather than crash.

Notes

The root case is the input or output of schema is empty and the corresponding field in graph is no-empty. So the input.back from schema will invalid.