fix: didn't check for empty sell

lib/ui/sell_page/widgets/sell_page.dart

@@ -140,6 +140,17 @@ class _SellPageState extends State<SellPage> {
                           children: [
                             IconButton(
                               onPressed: () {
+                                if (widget.viewModel.scannedBooks.isEmpty) {
+                                  ScaffoldMessenger.of(context).showSnackBar(
+                                    SnackBar(
+                                      content: Text(
+                                        "La vente doit comporter au moins un livre",
+                                      ),
+                                      behavior: SnackBarBehavior.floating,
+                                    ),
+                                  );
+                                  return;
+                                }
                                 if (double.tryParse(
                                       price.text.replaceFirst(",", "."),
                                     ) ==