Zend\Cache\Pattern\CallbackCache doesn't work with NULL

https://github.com/zendframework/zf2/blob/master/library/Zend/Cache/Pattern/CallbackCache.php#L51

<code>if (!isset($result[0])) {
 throw new Exception\RuntimeException("Invalid cached data for key '{$key}'");
}</code>

The cache has been saved and loaded correct but this condition returns false because <strong>isset</strong> returns false on <em>NULL</em> - should be replaced by <strong>array_key_exists</strong>

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