[Fusilli-plugin] Graph import has issues if graph name isn't set

Greetings!

In the graph import from hipDNN graph -> Fusilli graph, if the graph name isn't set, then an issue can occur.
hipDNN allows you to set an empty name on the graph, but due to the way flatbuffer serialization works, [this](https://github.com/iree-org/fusilli/blob/0ace845334e218b192728448d97c455807e91693/plugins/hipdnn-plugin/include/graph_import.h#L96) usage can fail.

I believe name can be a nullptr in situations where the user provides no name for the graph.

To replicate you can use this [sample](https://github.com/ROCm/rocm-libraries/blob/982cda7c788f7b2eb5270db85395b79c90fad728/projects/hipdnn/samples/convolution/ConvFprop.cpp#L46) from hipDNN with fusilli-plugin installed & the preferred engine Id set, (or add code to load specifically the fusilli-plugin).

```
graph->set_preferred_engine_id_ext(1001); // ID of fusilli-plugin
```