onnxruntime

[TensorRT EP] Add null_ptr check to avoid crash when running session which was failed to generate trt_engine previously (#21621)

### Description
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Add null_ptr check to avoid crash when running session which was failed
to generate trt_engine previously


### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->

Reported and verified by
https://github.com/microsoft/onnxruntime/issues/21567