I think if master has not slave's at this time then server must use "local" mode. But if he has >=1 slave then he must wait until slave accept transaction. And "synchronous_standby_names" select behaviour about how master shoud select which slave must be synchronous or not. And when this GUC is '*' master should use first in the list IMHO.
In docs you (developers) write
PostgreSQL does not provide the system software required to identify a failure on the primary and notify the standby database server.
But I think you don't have provide software to identify AND standby failure (only notification about this situation's) - and this master without slave is this failure (slave don't started)