Fix a -Wmaybe-uninitialized warning in cuckoocache_tests

Summary: The warning is raised by GCC 13. KeyType::data is accessed in CacheHasher::operator() so we should initialize it.

Test Plan:

ninja check-bitcoin-cuckoocache_tests

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15098