Correct for the stack trace not existing

Fixes this error:




```bash
node_modules/grunt-notify/node_modules/stack-parser/index.js:49
  lines.forEach(function(line) {
        ^
TypeError: Cannot call method 'forEach' of null
    at Object.parse (node_modules/grunt-notify/node_modules/stack-parser/index.js:49:8)
    at notifyException (node_modules/grunt-notify/tasks/notify.js:81:31)
    at Object.notifyHook (node_modules/grunt-notify/tasks/notify.js:125:16)
    at Object.hooked [as fatal] (node_modules/grunt/node_modules/hooker/lib/hooker.js:108:32)
    at process.uncaughtHandler (node_modules/grunt/lib/grunt.js:123:10)
    at process.EventEmitter.emit (events.js:117:20)
    at process._fatalException (node.js:272:26)
DEBUG: Program grunt code exited with code 7
```