make deleteWithObjects() work with empty containers

As is, deleteWithObjects() will fail if container is EMPTY. This is because an object count of zero means $secondsToWait = 0, which means $endTime = time(), which means WHILE loop never runs. Quickest solution was to change "time() __<__ $endTime" to "time() __<=__ $endTime" on line 192, but it makes more sense to just delete the container if it's empty.
Rather than this `if`-`else` construction, you can just return early from inside the `if`. So something like this:

`if ($numObjects === 0) {
    return $this->delete();
}`

// If timeout (seconds to wait) is not specified by caller, try to
// estimate it based on number of objects in container
// ...
```