WA: Fix adb wait-for-device command

Without -s argument, adb wait-for-device will make a call to
acquire_one_transport() with sinfo->serial.c_str() == "".
Waiting for acquire_one_transport() to be reworked to use std::string
rather than const char * for serial, work around this by passing NULL
when sinfo->serial is not initialized in host_service_to_socket().

Change-Id: Ifef53e0e82850364f5029ee36560376f2a4a5104
Signed-off-by: Leo Sartre <leox.sartre@intel.com>