abc9_ops: -write_box is empty, output a dummy box to prevent ABC error

passes/techmap/abc9_exe.cc

@@ -510,9 +510,8 @@ struct Abc9ExePass : public Pass {
 			}
 		}
 
-		// ABC expects a box file for XAIG
 		if (box_file.empty())
-			box_file = "+/dummy.box";
+			log_cmd_error("abc9_exe '-box' option is mandatory.\n");
 
 		rewrite_filename(box_file);
 		if (!box_file.empty() && !is_absolute_path(box_file) && box_file[0] != '+')

passes/techmap/abc9_ops.cc

@@ -569,6 +569,9 @@ void write_box(RTLIL::Module *module, const std::string &src, const std::string
 		module->attributes.erase(it);
 	}
 
+	if (ofs.tellp() == 0)
+		ofs << "(dummy) 1 0 0 0";
+
 	ofs.close();
 }