Toast: setting 'duration' to null results in toast not displaying at all

[gbruins-notours]

Describe the bug
The docs for the 'duration' property of the Toast component say: "Duration before dismiss in milliseconds, or null to never dismiss.", however setting a duration of null results in the toast not displaying at all.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://vue.chakra-ui.com/toast
2. In one of the "Editable Example" boxes, change "duration: 10000" to "duration: null"
3. Click the respective "Show Toast" button
4. The toast does not display.

Expected behavior
The toast should display indefinitely

Desktop (please complete the following information):

• OS: macOS High Sierra
• Browser chrome 86, firefox 81