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

[dimaryaz]

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.