Empty or missing values in vert files

[TomazErjavec]

The idea is that any empty values in the attributes to the <speech> tag in vertical files have as their value '-'. Currently this is not the case: sometime some values are empty (e.g. speaker_id="") or missing all together.
This needs to be corrected, mostly in parlamint-lib.xsl, but also in parlamint2xmlvert.xsl, and possibly (if the same thing happens in metadata TSV files) in parlamint2meta.xsl.

[TomazErjavec]

With the latest fixes, I regenerated all the vertical files and checked them for empty or missing attributes on speech, and it now seems ok, so, closing.