[lldb] Fix crash when launching in terminal

This patch fixes a crash when using process launch -t to launch the
inferior from a TTY. The issue is that on Darwin, Host.mm is calling
ConnectionFileDescriptor::Connect without a socket_id_callback_type. The
overload passes nullptr as the function ref, which gets called
unconditionally as the socket_id_callback.

One potential way to fix this is to change all the lambdas to include a
null check, but instead I went with an empty lambda.

Differential revision: https://reviews.llvm.org/D124535