Before, empty lines would get no prefix added. Now, empty lines have a
trimmed prefix added.

This little trick makes `indent` much more useful: you can now indent
with `"# "` without fearing creating trailing whitespace in the output
due to the trailing whitespace in your prefix.

When you indent with a prefix of pure whitespace, such as `" "` or
similar, you will see no difference. If you indent with a
non-whitespace prefix, you will now see it added to every line of your
input.