Brought back TAB to switch team/all chat when nothing has been typed yet

The new ALT hotkey from https://github.com/OpenRA/OpenRA/pull/6281 is driving me nuts. When you have not typed anything and therefore nothing can be auto-completed, this will allow you to still use it to switch team and public chat.
If you changed this to instead check `if (old text length == new text length) SwitchTeamChat()` then the toggle will also work when text has been entered.