Format: be future proof against fixes in SpiderMonkey

Modern JS engines (in Firefox and Chrome, as well as node) pass
undefined, not '', to the replaced function for groups that are not matched.
Be future proof against changes that might go unnoticed when
porting to SpiderMonkey 31/38/45/whatever that will be, and also
if people extract this code and use it against different engines.

(At least SM 34 has the right behavior, not sure about 31)

https://bugzilla.gnome.org/show_bug.cgi?id=770111