Fix LruCache, allow std:string caching

The default initalization for mNullValue uses 0 which is in the case of
a std::string TValue will invoke the contructor with undefined behavior
parameter.

Using an empty uniform initialization {} addresses the problem.

Test: Already tested in lrucache_test.cpp
Bug: 257127748
Change-Id: I37420ce8a16c99f3014538a0208d7e113870b1c7