[Enhancement] Support empty value "" to remove attribute

[t2ym]

Support the special empty value "" to remove attribute

var xliffOptions =
{ xliffStates: {
  'add'    : [ 'new' ],
  'replace': [ 'needs-translation', 'needs-adaptation', 'needs-l10n' ],
  'approve' : [ '""', 'needs-review-translation', 'needs-review-adaptation', 'needs-review-l10n', 
        '[!approved&&state:=""&&approved:=no]' ],
  'default': [ '', 'translated', 'signed-off', 'final', '[approved]' ]
} };