cache-db: fix empty to clear cache

In file included from ../src/cache/cache-db-test.cpp:21:
../src/cache/cache-db.h: In instantiation of ‘void dss::CacheDb<T>::empty() [with T = {anonymous}::TestEntry]’:
../src/cache/cache-db-test.cpp:59:5:   required from here
../src/cache/cache-db.h:131:5: error: ignoring return value of ‘bool std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::empty() const [with _Key = std::__cxx11::basic_string<char>; _Tp = {anonymous}::TestEntry; _Hash = std::hash<std::__cxx11::basic_string<char> >; _Pred = std::equal_to<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, {anonymous}::TestEntry> >]’, declared with attribute nodiscard [-Werror=unused-result]
  131 |     m_cache.empty();
      |     ^~~~~~~