Given two lists of equal length, l1 and l2, both consisting of sublists that are either empty or non-empty – but corresponding sublists are never non-empty – the task is to consolidate or “collapse” the lists to make a new list newList where

• if the nth element of l1 is an empty list, but the same element of l2 is not, then newList[[n]] takes that sublist from l2;
• if the nth element of l1 is non-empty, but the same element of l2 is an empty list, then newList[[n]] takes that sublist from l1;
• if the nth element of both l1 and l2 are empty lists, then so will be the same element of newList

I have some working code for such a function, but I’m curious if there is a simpler or faster way to execute this sort of task. Perhaps I’ve overlooked a built-in method? Any suggestions appreciated (even for making the question title more concise).

Collapse[l1_List, l2_List] := Module[      {newList, n, commonLength},      newList = {};      If[         Length[l1] === Length[l2],          commonLength = Length[l1]; n = 1;         While[             n <= commonLength,             If[                 (l1[[n]] === {}) && (l2[[n]] =!= {}),                 AppendTo[newList, l2[[n]]],                      If[                         (l1[[n]] =!= {}) && (l2[[n]] === {}),                         AppendTo[newList, l1[[n]]],                          If[                             (l1[[n]] =!= {}) && (l2[[n]] =!= {}),                             Print["Lists are not collapsible"],                             AppendTo[newList, {}]                         ];                      ];                  ];              n++          ];,          Print["Lists are incompatible"];      ];      If[         newList =!= {},         Return[newList]     ];  ]; 

Test cases

Collapse[{{}}, {{}}] (* {{}} *)  Collapse[{{}}, {{1}}] (* {{1}} *)  Collapse[{{}, {1}, {}}, {{}, {}, {a}}] (* {{}, {1}, {a}} *)  Collapse[{{}}, {{}, {}}] (* Lists are incompatible *)  Collapse[{{1}}, {{a}}] (* Lists are not collapsible *)