fix `HISTFILE` issue

[meganrogge]

This conditional wasn't entered when a $HISTFILE existed bc it was using -z instead of -f; thus, the HISTFILE wasn't getting set to the user's. https://github.com/microsoft/vscode/blob/4c0f7ff2a579a70c8e140096fdaeeeea79665360/src/vs/workbench/contrib/terminal/browser/media/shellIntegration-rc.zsh#L26-L28

to test, echo $HISTFILE in zsh - you'll see the VS Code tmp dir in the path in Insider's

in OSS based on this branch, you should see your $ZDOTDIR in the path instead

I will cherry-pick this to the release branch and reference the issue in that PR