Fix exception when calling init for server with no middleware

[k]

I was getting the error shown below:

 1  /home/k/pavlok-punctual/node_modules/pavlok-beta-api-login/index.js:50
 2         return !!app._router.stack.filter(function (layer) {
 3                             ^
 4
 5 TypeError: Cannot read property 'stack' of undefined
 6     at middlewareExists (/home/k/pavlok-punctual/node_modules/pavlok-beta-api-login/index.js:50:22)
 7     at Object.exports.init (/home/k/pavlok-punctual/node_modules/pavlok-beta-api-login/index.js:168:7)
 8     at Object.<anonymous> (/home/k/pavlok-punctual/app.js:5:8)
 9     at Module._compile (module.js:571:32)
10     at Object.Module._extensions..js (module.js:580:10)
11     at Module.load (module.js:488:32)
12     at tryModuleLoad (module.js:447:12)
13     at Function.Module._load (module.js:439:3)
14     at Module.runMain (module.js:605:10)
15     at run (bootstrap_node.js:427:7)

When running a simple server with no middleware, e.g.:

var pavlok = require('pavlok-beta-api-login');

pavlok.init("7f53e074cafe5151525a0934586bd39d4f7bc1156e67d53b862b9fa3dea1c6bd",
            "ff355d4688bc72b9641035a0edb8a8b569e67a305e32cf5fe329e9525b324513", {
    "verbose": true,
    "app" : app,
    "message": "Hello! Stay punctual or else...",
    "callbackUrl": "http://www.io.k33.me:3000/pavlok/result",
    "callbackUrlPath": "/pavlok/result",
    "successUrl": "/success",
    "errorUrl": "/error"
});

It started working after I added a simple middleware before the .init call, so I I added an if statement to cover this case.