Odd titlebar when toggling layout to windowed or stacked on empty workspace

## I'm submitting a…

[x] Bug

[ ] Feature Request

[ ] Documentation Request

[ ] Other (Please describe in detail)

## Current Behavior
When pressing $mod + W or $mod + S (by deafult) and toggling layout to tabbed or stacked on empty workspace (no apps are opened) and opening some app we get odd titlebar with name like "i3: T[urxvt]" or "i3: S[firefox]" for example

## Expected Behavior
No titlebar when just switching to tabbed or stacked layout mode and opening one app.

## Reproduction Instructions
On empty workspace:
1. $mod + W or $mod + S
2. Open any app
3. ???
4. PROFIT
5. You have one odd titlebar

## Environment

Output of `i3 --moreversion 2>&-`:

Binary i3 version:  4.19.2 (2021-02-27) © 2009 Michael Stapelberg and contributors

Running i3 version: 4.19.2 (2021-02-27) (pid 594)

Config file

Default

Logfile

Not needed. Bug is visual and reproduceable