R1:c1d86da5b209

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…