fix: Check train_data_file None

app.py

@@ -803,7 +803,7 @@ class GGUFConverterUI:
                 method=q_method,
                 use_imatrix=use_imatrix,
                 imatrix_method=imatrix_q_method,
-                train_data=train_data_file.name,
+                train_data=train_data_file.name if train_data_file else None,
                 quant_embedding=quant_embedding,
                 embedding_tensor_method=embedding_tensor_method,
                 leave_output=leave_output,