fix: gracefully use default for assets

[wesleycho]

• If the assets key is not set in the options object (i.e. the files array has all of the listed assets desired), set to default to an empty array

This gracefully handles the situation where one is not using options.assets - Grunt expects it to be an array of strings, so it being undefined causes Grunt to error.

[benhoIIand]

Hi @wesleycho. What's the use case for this? When would this task be run without the need to provide assets to be busted?

[wesleycho]

@hollandben my company specified the assets with the files array - upon upgrading grunt to 1.0, we ran into this regression in this library where it would fail without the options asset key being an array.

[benhoIIand]

The assets option is a hard requirement for the plugin to work (from v1 onwards). It's worth having a read through the documentation and #147 first to make sure you've got the correct configuration