Gives time to the user to enter an actual value in m2o in list views.
Before this commit, emptying a many2one field (selecting all > BACKSPACE or DELETE)
triggered instantly a field_change. This was an issue in multi edition as a field
changed triggered instantly a save on selected records without giving time to the
user to enter a new value.
Now, many2ones only trigger a fieldChange when selecting a new valid value.