Initialize $cache_key variable in CelerityResourceController.php

Summary:
$cache_key is unconditionally called in $cache->setKey($cache_key, $data) but is only defined if the previous condition if ($is_cacheable && $is_locally_cacheable && !$dev_mode) was true.
Thus initialize the variable to avoid a theoretical exception. (No additional null check is needed as $cache gets defined in the same condition.)

Test Plan: Carefully read the code.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25642