Make SBDebugger::CreateTargetWithFileAndArch work with lldb::LLDB_DEFAULT_ARCH

Second try, handling both a bogus arch string and the "null file & arch" used
to create an empty but valid target.
Also check in that case before logging (previously the logging would have
crashed.)

GitOrigin-RevId: 7636b1f6efd56063adcde4f908740660365462a5
2 files changed