Notice prevents update check when error reporting is on

### Issue
When running Joomla on my local host or any other php environment that has the maximum or development error reporting enabled, the ajax request that the control panel received to determine whether there are updates available or not contains a php parser notice that prevents the JSON response to be interpreted correctly.

### Solution
The error happens in Line 97 of the libraries/joomla/adapters/extension.php. It needs to be checked whether `$this->currentUpdate` is null before checking it's values in the if statement.