Allow empty zip files. Require a line break at the end.

An empty zip file is completely valid, but the current mod_zip syntax
does not support them. This comes up in cases where e.g. a user clicks
"Download this folder" - and the backend needs to special-case empty folders
and use different code to generate an empty .zip file.

This is technically a breaking change - though the documentation does not
say anything about trailing line breaks, and it seems unlikely that
anyone would actually rely on the last line break being optional.