Completion notifications do not work when the default shell is ZSH

## Prerequisites
- [X] I have searched open and closed issues for duplicates.

## Describe the bug

### To Reproduce
Installed ZSH via the oh-my-zsh install script.
Run the command `sleep 10`, open another tab, wait 10 seconds.

### Expected behaviour
Old tab should get a green check icon after 10 seconds and play a notification ringtone.

## Platform Information
OS: elementary OS 5.1 hera
Kernel: x86_64 Linux 5.0.0-37-generic
Shell: zsh 5.4.2
Resolution: 1920x1080
DE: GNOME
WM: Mutter(Gala)
WM Theme: elementary
GTK Theme: elementary [GTK2/3]
Icon Theme: elementary
Font: Open Sans 9
CPU: AMD PRO A12-9800B R7, 12 COMPUTE CORES 4C+8G @ 4x 2.7GHz [73.0°C]
GPU: AMD/ATI Wani [Radeon R5/R6/R7 Graphics]

Please check what applies:
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- [ ] I'm using the latest version from git that I've manually compiled
- [X] I'm using the latest released stable version