Fix change to old shell on uninstall (#8430)

If there's no `~/.shell.pre-oh-my-zsh`, don't assume the default choice
is Bash.  In fact Zsh is the default shell for macOS since Catalina
(10.15) [1], yet users of other OSes have likely to have Bash as their
default.

This commit fix issue #8252

[1] https://support.apple.com/en-us/HT208050