terminal: Don't output @ for ASCII NUL

Fixes visual bell in screen.
diff --git a/clients/terminal.c b/clients/terminal.c
index dd35ceb..782f087 100644
--- a/clients/terminal.c
+++ b/clients/terminal.c
@@ -1802,6 +1802,8 @@
 	case '\x0F': /* SI */
 		terminal->cs = terminal->g0;
 		break;
+	case '\0':
+		break;
 	default:
 		return 0;
 	}