Pasting sequence which contains only gaps generates a trivial undo action in the Msa Editor

Scenario:

1. Open "data/samples/CLUSTALW/COI.aln".
2. Copy a sequence which contains only gaps to the clipboard.
3. Paste the clipboard data to the alignment.
   Expected state: nothing happens, the undo/redo stack hasn't been modified.
   Current state: nothing happens, but a new trivial modification action has been inserted into the undo/redo stack.