[VitisAI] fix dangling pointer (#27949)
### Description
set the pointer to nullptr immediately after `UnloadDynamicLibrary`.
### Motivation and Context
After unload library, set the function pointer to nullptr to avoid a
dangling pointer. Otherwise, the following scenario may cause errors:
```
RegisterExecutionProviderLibrary()
SessionOptions::AppendExecutionProvider_VitisAI()
```
In this scenario, the OrtVitisAIEpAPI will call `initialize_vitisai_ep`
once but call `deinitialize_vitisai_ep` twice. During deinitialization,
functions `deinitialize_onnxruntime_vitisai_ep` are invoked, which leads
to errors.
