[QNN-EP] Fix data type check to skip optional I/Os (#25514)
### Motivation and Context
Fix data type check to skip optional I/Os.
Optional inputs/outputs would have empty name, and it's valid ONNX syntax. Without this fix, any model with optional inputs/outputs would fail the check due to missing protobuf fields.
Without this fix, we'd hit error fetching `elem_type` from protobuf.
```
2025-07-22 11:14:40.117740035 [I:onnxruntime:, qnn_execution_provider.cc:740 GetSupportedNodes] Validation FAILED for 1 nodes in NodeUnit (Pad) :
Operator type: Pad Node name: /blocks.4/Pad Node index: 176 REASON : The tensor doesn't have elem_type.
```
