Update index.js
Update index.js

If there are no matching source files for a test then collector.add(...) tries to add an undefined object. since the collector just loops through the keys it should just skip over the supplied empty object from the  || {}.
nigampatel/gulp-istanbul,jedmao/gulp-istanbul,duereg/gulp-coffee-istanbul,cb1kenobi/gulp-babel-istanbul,kjvalencik/gulp-babel-istanbul,danieljoppi/gulp-istanbul,andrew-minted/gulp-istanbul,ToPeter/gulp-istanbul,markwylde/gulp-istanbul-plus,kkglobes/gulp-babel-istanbul,mulyoved/gulp-istanbul,SBoudrias/gulp-istanbul