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 ConversionHelperAidl.

Bug: 295394788
Test: make checkbuild
Change-Id: Ic466986bdf850a44c30c919450fd8b92ac68758d