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