exists() requires a message or else QUnit complains

If `exists()` is the only test which runs and a message is omitted, QUnit will complain that at least 1 assertion was expected.

``` javascript
QUnit.test("The input exists", function () {
F("input").exists();
});
```

The following will report that 1 assertion was successful:

``` javascript
QUnit.test("The input exists", function () {
F("input").exists('The input exists');
});
```

Start at the exists() entry point and trace how it reports assertions to QUnit. Reproduce the issue with the two QUnit.test examples in the report; done means exists() without a message records one successful assertion and QUnit no longer complains about the expected assertion count.

Written by the indexing model from the issue text.