Prevent TypeError when the page being inspected has no images.

This simply sets the number to zero when no images are found.

It's a pretty simplistic solution, but it definitely works. I didn't add a test but I'm happy to do so if you think it would be useful.

```text
$ ./bin/cli.js http://brycebaril.com
Your image weight:
0 B
Average image weight on the web:
1.2 MB
Thanks for keeping the web fast <3
```