File Error in extract process

Looking over the documentation, I do not see anything referring to options.extra being required. Given this, I've wrapped the options.extra file read in a logic check to avoid a failure, as shown below, when you don't set it.

madzak$ grunt xliff
Running "xliff:export" (xliff) task
Warning: Unable to read "undefined" file (Error code: ENOENT). Use --force to continue.