fixing-plugins: If the [data-autoplay] attribute is missing, do not run ...

If the `data-autoplay` attribute is not included `( autoplay !== null && autoplay !== false )` will evaluate to `true` and the autoplay plugin will run if included on the page.
If the attribute is missing, `$.attr('data-autoplay')` will return `undefined`.
I set autoplay to return null so in the case that the attribute is not there, then the conditional statement will evaluate to false and the plugin will not run.