Remove `is_null()` in favor a `null` comparation

You should use strict comparison operator `===`  instead of `==`.
for zf2 consistency, `  if ($this->_storageClient === null) { `  is more preferable.
https://github.com/zendframework/ZendService_WindowsAzure/blob/master/library/ZendService/WindowsAzure/Storage/Blob/Stream.php#L77