Don't use String8::empty

With the current definition, it's an alias of default constructor. This
has very different meaning from std::string empty, especially that
static method String8::empty can still be called from instance context.

One of such bugs are present in in ParametersUtil.

Bug: 295394788
Test: make checkbuild
Change-Id: I9116a0c42a6a9e94116eaaaa0fd60568db428838