7.4 deprecation removal on array_key_exists

[Kalyse]

(Second attempt, reliased I didn't even look at last PR)

Anyway, quite straightforward,

Currently the $return is set based on

$return = $this->useContainerObject ? new KeyValueContainer() : array();

This is being changed to use isset due to php 7.4 deprecation on objects when using array_key_exists