[VitisAI] Fix OrtShapeInferContext for optional inputs

[zpye]

Description

When there is an optional input (empty input type) in the OrtShapeInferContext construction, use undefined data type and empty shape as a placeholder.

Motivation and Context

VitisAI EP may add nodes with optional inputs during graph optimization to meet the requirements of AMD AI compilers.

This fix may help other execution providers to improve the graph optimization process.

[Copilot]

Pull Request Overview

This PR fixes the OrtShapeInferContext constructor to properly handle optional inputs by adding a null check for input types. When an input type is null (indicating an optional input), it now creates a placeholder with undefined data type and empty shape instead of attempting to process a null pointer.

• Added null check for input_type before processing tensor type information
• Added fallback logic to create placeholder type and shape info for optional inputs

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