"Add new values to taxonomy vocabulary instead of overwriting" doesn't work, if no terms have been previously assigned

Existing behaviour

You need

• a node type that contains a free-taging taxonomy field
• some nodes of this type:
  a few of them should have some tags for this taxonomy vocabulary,
  a few others should not.
• to go to admin/content, enable the nodes' checkboxes and select "change value" from dropdown menu
• in the screen that appears, go and enable the checkbox for these Tags vocabulary, then add some values and then
• check " Add new value to Tags, instead of overwriting the existing values. "
• view the affected nodes

You will notice that the tag will be added into the nodes that have already some tags, but won't be added if the node's tags list is empty.

Desired behaviour

It would be expected that tags should be added both when there are already terms in the node or not.