[perl #114498] lslice returning empty vs undef

Formerly, list slice would return an empty list if all the indices
fell outside the actual list, but would return ‘undef’ for every index
specified if but one of them fell within the list.

This was not implemented according to the original design, according
to which list slice would give an empty list (for a non-zero number of
indices) only if the list on the left-hand side were empty.

This commit rectifies that.  See ticket #114498 for the discussion.
perldata: document corrected list slicing behaviour