SDLXLIFF: <seg-source> with no <mrk> still generates a segment

SDLXLIFF uses to keep track of sources. Normally, is used to identify segments; markup (such as codes) that don't fall within a pair are correctly exposed as non-segment TextParts by the filter.

However, in some cases Studio may produce a trans-unit that consists only of codes and contains no actual segments. I've attached a reduced testcase that shows this -- this file contains one trans-unit, but will not display any segments in Studio, because there are no 'd sections of the . Our XLIFF filter will report one gigantic, garbage TextUnit containing several hundred codes.

Basically, if there are no sections in the , the correct behavior for SDLXLIFF is to not return any segments, only non-segment TextParts. What I'm worried about is whether this assumption is safe to make for other tools that may be using seg-source.