ncm2: Set 'abbr' to empty string if not set

While setting up the new combine matcher (which is great, thanks!) for vimtex, I noticed that ncm2 initializes the abbr field to word in https://github.com/ncm2/ncm2/blob/master/pythonx/ncm2.py#L85. Is there a specific reason for this? This prevents me from matching word more strictly than abbr; for this purpose,

 if 'abbr' not in e or type(e['abbr']) != str:
            e['abbr'] = ''

would be needed. (This would also be more consistent with the other fields.)