[QNN-EP] Fix ONNX context model helper. (#24271)
### Description
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Fix the bug where the QNN EP generates an ONNX model with EP Context and fails to run.
### Motivation and Context
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- If it fixes an open issue, please link to the issue here. -->
When generating an ONNX model with QNN EP context where the input is scalar, the shape is not set, resulting in a null pointer and causing the subsequent run to fail.
