[lldb] Fix DomainSocket::GetSocketName for unnamed sockets

getpeername will return addrlen = 2 (sizeof sa_family_t) for unnamed
sockets (those not assigned a name with bind(2)). This is typically true
for client sockets as well as those created by socketpair(2).

This GetSocketName used to crash for sockets which were connected to
these kinds of sockets. Now it returns an empty string.