/objectbrowser - Fix handling multichoice fields
/objectbrowser - Fix handling multichoice fields

* When a multichoice field is emptied, it will now
  get the value "null" instead of removing the attribute.

* When a multichoice field is emptied, remove any
  + ! or - prefixes on the field

Fixes #290

opinkerfi/adagios,zengzhaozheng/adagios,opinkerfi/adagios,opinkerfi/adagios,kb2ma/adagios,kaji-project/adagios,kaji-project/adagios,kaji-project/adagios,kb2ma/adagios,opinkerfi/adagios,zengzhaozheng/adagios,kaji-project/adagios,zengzhaozheng/adagios,kb2ma/adagios,kb2ma/adagios,zengzhaozheng/adagios