- BaseObjectBuilder is used to look up existing objects
  like labels during import, instead of creating one
- For example, if a label is used on an epic and a board,
  we do not want to create 2 labels and instead reuse an
  one
- If a label is exported with description: '', and then imported
  back, rails will convert description to be nil
- Later on, when we try to identify a label BaseObjectBuilder
  uses description: '' as one of the where causes and
  nil != '', hence label lookup fails and the relation creation
  fails as well
- This MR converts any empty strings to nil inside BaseObjectBuilder
  so it does not happen anymore