Follow up to: https://github.com/zed-industries/zed/pull/10986

However, I have set this to have a default behavior of 'auto': matching
the current platform's conventions, rather than a default value of
'off'.

fixes https://github.com/zed-industries/zed/issues/5322.

Release Notes:

- Changed the behavior of `workspace::CloseActiveItem`: when you're
using macOS and there are no open tabs, it now closes the window
([#5322](https://github.com/zed-industries/zed/issues/5322)). This can
be controlled with a new setting, `when_closing_with_no_tabs`, to
disable it on macOS, or enable it on other platforms.