Fix Notification raises a MUI warning when message is a string

[slax57]

Fix #8664

Notes

Didn't add a test cause I'm not sure it would make sense to have a test ensuring MUI does not raise any warning (as it's pretty specific). But I can add one if you guys think otherwise.

The fix can be tested with the Basic story of the Notification storybook.

[magnattic]

Thank you! This helps us a lot. Minor nitpick, the title of the PR should probably be

Fix Notification raises a MUI warning when message is NOT a string

right?

[slax57]

@magnattic No, it's the other way around. When message is not a string, it is a React Element, and in this case the Snackbar throws no error because it is supported.
When message is a string, then instead of passing no child to Snackbar we passed false, which caused the warning.
So the fix actually applies when message IS a string.