For Brave importer profile writer added a nullptr optional for tab group.

Chromium change:

https://chromium.googlesource.com/chromium/src/+/81999fd6a771e38cccc487364728ba91f5fc17f2

commit 81999fd6a771e38cccc487364728ba91f5fc17f2
Author: Collin Baker <collinbaker@chromium.org>
Date:   Wed Jun 5 23:12:27 2019 +0000

    Save and restore tab groups

    This implements basic session restore functionality for tab
    groups. Currently, only whole browser session restore is supported;
    restoring a window or a tab from the history menu won't restore
    groups.

    Bug: 905491