Disabeling ServiceWorker is not possible anymore

[mojoaxel]

The docs state that you could set ServiceWorker to null or false to disable it.

Since 39acf1f this does not work anymore. The check if ServiceWorker.force is true breaks if ServiceWorker is null or false:
https://github.com/NekR/offline-plugin/blame/master/tpls/runtime-template.js#L4

PR #388 fixes this issue.