Support for empty language content

[needim]

In current version of minibed you need to provide files. If you don't provide any file for a language it's tab doesn't show.

e.g

files: {
  html: [],
  js: ['example.js']
}

In this case minibed only shows JavaScript tab. But I want HTML tab too.

We can check for null, 'empty' or 'blank' values and consider as a blank content.