Allow no comments for imported strings

Developers need to provide comments for translators, so zz.lproj strings must have comments for each entry. However, when importing back, we should allow strings with no comments coming from our favourite localisation service, as translators may put records without comments.

As zz.lproj is the source-of-truth for keys AND comments (while the real languages are the source-of-truth for values), we know for sure that the final file won't have entries with no comments.