source with no text in .sdlxliff should be kept in skeleton

[Former user]

Original [issue 56](https://code.google.com/p/okapi/issues/detail?id=56) created by @ysavourel on 2009-04-22T03:54:22.000Z:

The .sdlxliff files may have tons of trans-unit with white spaces only (like line-breaks) but not set a structural no-translate. They get extracted and because xml:space is not set to preserve they may get normalized. The xliff filter needs to keep those tu in the skeleton.

[Chase Tingley]

I think this is the same as issue #466.

[ysavourel]

Duplicate of #466.