Check fails if a `.vue` file contains no `<script>` tag

[Twiknight]

If a .vue file contains no <script> tag, eslint would throw following error:

Module build failed: TypeError: Cannot read property 'forEach' of undefined

This is the consequence of sending an empty array as default text set to eslint.

preprocess: function (text, filename) {
    ...
    return []
  }

https://github.com/Twiknight/eslint-plugin-vue/blob/master/lib/processors/vueProcessor.js#L17

[Twiknight]

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