Allow truffleruby to install in an existing empty directory

The check to prevent deleting user files during clean_prefix_path by disallowing
installs into existing directories that weren't earlier truffleruby installs broke
asdf installs, as asdf always creates an empty directory before installing.

This fix adds a second exception to the existing directory case when that directory
is empty. The check is done via testing the length of ls output. The logic was
manually tested against non-existing directories, existing empty directories,
existing non-empty directories in general, and existing non-empty directories
that contained a file named "bin/truffleruby".

I also copied the file over my existing asdf version of ruby-build and made sure that
it successfully installed truffleruby.