Fix bag (string-array, etc) behavior with overlays.
We used to replace elements in the default with elements from the
overlay. This change causes us to empty the array first so if the
overlay array is smaller we don't end up with elements from the
default array showing through at the end of the final result.
Ex: [A,B,C] default with [D] overlay should give [D] but used to
give [D,B,C]
BUG=1754390
Automated import of CL 144950
