Don't die when dir is empty or has no recognized lang files

[adz]

Hello!

I (perhaps) fixed a little bug where the .unwrap() can be None when returning total_lines. Happened for me when I ran in my projects folder when this repo was checked out in it: https://github.com/adz/xps9360 - i think nothing in there is recognized, and skipped maybe?

Anyway, I do not know Rust at all aside from reading 1/2 the tutorial, but had a go at it!