tiny-cloud main shows error if default user does not exist and no ssh key is provided

There is no reason to show any error about missing default user if there is no ssh key.

   ++ missing_userdata: starting
missing_userdata: no user-data found
   ++ missing_userdata: done
   ++ set_hostname: starting
set_hostname: no hostname set
   ++ set_hostname: done
   ++ set_ssh_keys: starting
set_ssh_keys: failed to find user alpine
   ++ set_ssh_keys: failed

We should simply return early in case of missing ssh key.