Fixes the following error:

      TypeError: Reduce of empty array with no initial value
          at Array.reduce (native)
          at Function.upgradeInstructions (/usr/src/app/lib/config_upgrader.js:272:20)
          at Object.<anonymous> (/usr/src/app/bin/eslint.js:239:37)
          at Module._compile (module.js:556:32)
          at Object.Module._extensions..js (module.js:565:10)
          at Module.load (module.js:473:32)
          at tryModuleLoad (module.js:432:12)
          at Function.Module._load (module.js:424:3)
          at Module.runMain (module.js:590:10)
          at run (bootstrap_node.js:394:7)

I couldn't find a good initial value to give to `reduce` to obtain the
correct functionality. There's no reason to reduce anyway if there are
no reports.