Update index.js

Fixes the case when you pass a bunch of controller files but only some of them contain jsdoc annotations.
eg: 

    app.use(swagger.init(app, {
        apiVersion: '1.0',
        ...,
        apis: fs.readdirSync('./controllers').map(function (filename) {
            return path.join(controllersPath, filename);
        }),
        ...
    }));