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.
Also, this style of conditional is not PSR-2 compliant, so will fail our syntax linter :smile: You can fix these by running:


```
./vendor/bin/php-cs-fixer fix --level psr2 ./lib/OpenCloud/ObjectStore/Resource/Container.php
```