Revert "Don't raise an exception when bin/sandi_meter is called without argument"

This reverts commit 588fd2fcc9c5214f11ac5788fb1b5559b2ffa7b3.
Avoid exception when bin/sandi_meter is called without argument

Without giving an existing path to the script it raised a Type Error (lib/file_scanner.rb:11:in `directory?': no implicit conversion of nil into String (TypeError)).
It now defaults to the current directory.