[builder] parse_glyphs_filter: make 'args' and 'kwargs' element optional
[builder] parse_glyphs_filter: make 'args' and 'kwargs' element optional

No need to write them to the lib plist when they are empty.
ufo2ft will check and use them only if emtpy.

'args' defaults to empty tuple, 'kwargs' empty dict.

googlei18n/glyphsLib,googlefonts/glyphsLib