Fixed rusty-paths.nu entering into rust folder

[TheCodeLamp]

When starting a new shell session inside a rust folder you'd get this error:

This pr fixes this issue.

Entering a folder without having a previous folder happen when you for example start a new terminal or starts a multiplexer already inside that folder.

[Bahex]

I think it makes more sense to just return early when $before == null, as there is no reason for the second hook to run on startup.

[TheCodeLamp]

Yes, you are correct. I'll fix this by just adding and ($before | is-not-empty) to the end of the condition.

[TheCodeLamp]

Is this better?

[fdncred]

Thanks