AbstractMarkupFilter produces placeholder-only segments

Created by: Anonymous

Original issue 336 created by @ysavourel on 2013-05-08T17:52:41.000Z:

See attached HTML testcase. This file contains no translatable text, but produces 5 trans-units when converted to XLIFF with tikal. In all cases, the TUs contain just a single placeholder, without even any whitespace.

These TUs are of questionable quality. I would propose that the filter shouldn't create TUs that consist solely of a single placeholder. I'm unsure about whether it should be an option. (Conceivably, somebody might want to do something with them, but at best I think that's a very small use case.)