commandline: Don't complain if insert mode has nothing to insert

That's just, like, a normal thing to happen.

We can return false, but it's not an error worth complaining about.

See #6931.

src/builtin_commandline.cpp

@@ -340,11 +340,8 @@ int builtin_commandline(parser_t &parser, io_streams_t &streams, wchar_t **argv)
     }
 
     if (append_mode && !(argc - w.woptind)) {
-        streams.err.append_format(
-            BUILTIN_ERR_COMBO2, cmd,
-            L"insertion mode switches can not be used when not in insertion mode");
-        builtin_print_error_trailer(parser, streams.err, cmd);
-        return STATUS_INVALID_ARGS;
+        // No tokens in insert mode just means we do nothing.
+        return STATUS_CMD_ERROR;
     }
 
     // Set default modes.