In the editmode sidebar, pasting a particular vertex's weight in a
single group was not behaving the same as copying for all groups [in
that the former dis not copy to unassigned vertices whereas the later
did copy to unassigned vertices].

This behaves like this since the introduction in rB70fd2320c8d2: Added polished Vertex Weights Panel (properties sidebar), but
there does not seem to be a good reason for this?

Now make this consistent and use BKE_defvert_copy_index in both cases
(instead of earlying out if unassigned, this will make sure this will
also copy to unassigned).