Cater for possibility of el not existing

[fergaldoyle]

While writing a drag and drop vue sortable directive which uses jQuery UI sortable (both same list and connected list sortable), I found vue was throwing el undefined errors a lot with various actions, e.g. dragging the first item in a list then updating the underlying array model. The element had been removed by jQuery while vue was expecting it to exist.

By checking for the existance of el, and returning and empty object if it does not exist, allow these two dom manipulation libraries play nicely together.

[yyx990803]

This will likely cause the list to go out of sync. I need to see a demo of jQuery UI sortable working properly with this change.

[fergaldoyle]

Here is the sortable directive with stock vue.js (throws errors)
http://jsfiddle.net/fergal_doyle/5Lrtte9v/7/

And here is the same directive but with a built of vue.js including the change I made in this PR:
http://jsfiddle.net/fergal_doyle/5Lrtte9v/6/

You can drag and drop items, use methods to reverse and add items to the model in any combination and the list stays in sync.

[yyx990803]

I finally got time to investigate this and it seems to indeed work! One thing here is that we can just if (!el) return, no need for the empty object.

[fergaldoyle]

I've made the changes, no empty object now being returned.

[yyx990803]

Thanks!