Inconsistent results when passing empty jQuery object to _.isEmpty

I am noticing some strange behavior when running _.isEmpty on an empty jQuery object. At first, _.isEmpty returns false, but if I add an empty collection to it the collection, it stays empty, but _.isEmpty returns true after that. Here is an example:

```
var result = $();
console.log("before", _(result).isEmpty(), result.length); // before true 0
result = result.add($("p"));
console.log("after", _(result).isEmpty(), result.length); // after false 0
```

This is on a page with no paragraphs, so the result should stay empty after adding `$("p")`. Is this a bug in Underscore, or is it in jQuery, or am I using isEmpty wrong? It seems like adding the empty collection creates some new properties in the result (length, prevObject, context), but it seems confusing that isEmpty sometimes returns true on empty array-like objects, and sometimes returns false.

I would try to diagnose the problem myself but I am busy. Here is a fiddle: http://jsfiddle.net/56kaw/

Start with Underscore's _.isEmpty behavior and reproduce the before/after example from the issue using an empty jQuery object and result.add($('p')). Inspect the resulting length, prevObject, and context properties, then determine whether the inconsistency is in Underscore or jQuery. Done means a confirmed cause and a regression test or clear resolution.

Written by the indexing model from the issue text.