Empty file causes build to fail with exception

[bcbailey]

If one of the assets is an empty file, carpenter will throw an exception and fail the build.
https://github.com/godaddy/carpenterd/blob/master/lib/constructor/factory.js#L443-L462

Error:
carpenter-sbjwi | {"level":"error","message":"Build 72af99d1-34f4-4dd5-acf7-d5f14f1f67fc failed {"message":"Cannot read property 'length' of undefined","stack":"TypeError: Cannot read property 'length' of undefined\n at Factory.stock (/usr/local/carpenter/node_modules/carpenterd/lib/constructor/factory.js:460:79)\n at Function.ran (/usr/local/carpenter/node_modules/carpenterd/lib/constructor/factory.js:293:15)\n at done (/usr/local/carpenter/node_modules/carpenterd/lib/constructor/workers/webpack.js:38:10)\n at Walker. (/usr/local/carpenter/node_modules/carpenterd/lib/constructor/workers/webpack.js:66:26)\n at Walker.g (events.js:260:16)\n at emitNone (events.js:67:13)\n at Walker.emit (events.js:166:7)\n at Walker._wNext (/usr/local/carpenter/node_modules/walk/lib/walk.js:277:10)\n at Walker._wNext (/usr/local/carpenter/node_modules/walk/lib/walk.js:272:17)\n at Walker._wOnEmitDone (/usr/local/carpenter/node_modules/walk/lib/walk.js:124:8)","name":"Error","event":"error","description":"Cannot read property 'length' of undefined","stacktrace":["TypeError: Cannot read property 'length' of undefined"," at Factory.stock (/usr/local/carpenter/node_modules/carpenterd/lib/constructor/factory.js:460:79)"," at Function.ran (/usr/local/carpenter/node_modules/carpenterd/lib/constructor/factory.js:293:15)"," at done (/usr/local/carpenter/node_modules/carpenterd/lib/constructor/workers/webpack.js:38:10)"," at Walker. (/usr/local/carpenter/node_modules/carpenterd/lib/constructor/workers/webpack.js:66:26)"," at Walker.g (events.js:260:16)"," at emitNone (events.js:67:13)"," at Walker.emit (events.js:166:7)"," at Walker._wNext (/usr/local/carpenter/node_modules/walk/lib/walk.js:277:10)"," at Walker._wNext (/usr/local/carpenter/node_modules/walk/lib/walk.js:272:17)"," at Walker._wOnEmitDone (/usr/local/carpenter/node_modules/walk/lib/walk.js:124:8)"],"branchId":[14]}"}