[TensorRT EP] Fix nullptr check (#16468)
### Description
Fix the nullptr check so that it would check the actual existence of
engine/context
(Currently, it checks the address of unique_ptr, which is always not
null. Thx @jslhcl for pointing that out)
> A quick recall of struct
[trt_state](https://github.com/microsoft/onnxruntime/blob/main/onnxruntime/core/providers/tensorrt/tensorrt_execution_provider.h#L104):
> ```
> std::unique_ptr<nvinfer1::ICudaEngine>* engine = nullptr;
>std::unique_ptr<nvinfer1::IExecutionContext>* context = nullptr;
>```
### Motivation and Context
https://github.com/microsoft/onnxruntime/issues/15982
The incorrect check couldn't stop TRT EP from loading incompatible
engine cache on purpose, which invokes unhandled exception
