Check for mTheRealServiceManager nullptr in getInterfaceDescriptor

It may be nullptr, so don't dereference it. Return an empty String16 in
this case because the binder is not backed by anything.

Flag: EXEMPT bug fix
Test: out/host/linux-x86/fuzz/x86_64/binder_parcel_fuzzer
Bug: none
Change-Id: I5043b6028812205e78405d9e47d0947a16279573