wait-online: request that at least one managed online interface exists

Fixes a regression caused by ab3aed4a03.

I thought the commit does not cause any severe regression. However,
drivers for network interfaces may be loaded later. So, we should wait
if no network interface is found.

Fixes #27822.

test/test-network/systemd-networkd-tests.py

@@ -1157,10 +1157,6 @@ class WaitOnlineTests(unittest.TestCase, Utilities):
     def tearDown(self):
         tear_down_common()
 
-    def test_wait_online_all_unmanaged(self):
-        start_networkd()
-        self.wait_online([])
-
     def test_wait_online_any(self):
         copy_network_unit('25-bridge.netdev', '25-bridge.network', '11-dummy.netdev', '11-dummy.network')
         start_networkd()