With no files in the Qif importer it is possible to proceed

On the page 'Qif files you have loaded' it is possible to unload files
and if all files are unloaded it was still possible to proceed so
disable the next button if no files are in the list.

gnucash/import-export/qif-imp/assistant-qif-import.c

@@ -2101,6 +2101,7 @@ gnc_ui_qif_import_account_prepare (GtkAssistant  *assistant, gpointer user_data)
     gint num = gtk_assistant_get_current_page (assistant);
 
     SCM  check_from_acct = scm_c_eval_string ("qif-file:check-from-acct");
+
     if (wind->ask_date_format && wind->date_format)
         qif_import_reparse_dates (wind);
    /* Determine the next page to display. */
@@ -2251,12 +2252,19 @@ update_file_page (QIFImportWindow * wind)
     GtkTreePath *path;
     GtkTreeRowReference *reference = NULL;
 
+    GtkAssistant *assistant = GTK_ASSISTANT(wind->window);
+    gint num = gtk_assistant_get_current_page (assistant);
+    GtkWidget *page = gtk_assistant_get_nth_page (assistant, num);
+    gint num_of_files = 0;
+
     /* clear the list */
     view = GTK_TREE_VIEW(wind->selected_file_view);
     store = GTK_LIST_STORE(gtk_tree_view_get_model (view));
     gtk_list_store_clear (store);
     qif_file_path = scm_c_eval_string ("qif-file:path");
 
+    gtk_assistant_set_page_complete (assistant, page, FALSE);
+
     while (!scm_is_null (loaded_file_list))
     {
         gchar *row_text    = NULL;
@@ -2292,6 +2300,12 @@ update_file_page (QIFImportWindow * wind)
         }
         gtk_tree_row_reference_free (reference);
     }
+
+    /* get the number of files in the list */
+    num_of_files = gtk_tree_model_iter_n_children (GTK_TREE_MODEL(store), NULL);
+
+    if (num_of_files > 0)
+        gtk_assistant_set_page_complete (assistant, page, TRUE);
 }