feat: lookup_host_with_cache(): Don't return empty address list (#7596)

All users of this function expect a nonempty list to be returned, otherwise they fail with hard to
understand errors like "No connection attempts were made", so it's better to fail early if DNS
resolution failed and the cache doesn't contain resolutions as well.

python/tests/test_1_online.py

@@ -1295,16 +1295,17 @@ def test_configure_error_msgs_invalid_server(acfactory):
         ev = ac2._evtracker.get_matching("DC_EVENT_CONFIGURE_PROGRESS")
         if ev.data1 == 0:
             break
+    err_lower = ev.data2.lower()
     # Can't connect so it probably should say something about "internet"
     # again, should not repeat itself
     # If this fails then probably `e.msg.to_lowercase().contains("could not resolve")`
     # in configure.rs returned false because the error message was changed
     # (i.e. did not contain "could not resolve" anymore)
-    assert (ev.data2.count("internet") + ev.data2.count("network")) == 1
+    assert (err_lower.count("internet") + err_lower.count("network")) == 1
     # Should mention that it can't connect:
-    assert ev.data2.count("connect") == 1
+    assert err_lower.count("connect") == 1
     # The users do not know what "configuration" is
-    assert "configuration" not in ev.data2.lower()
+    assert "configuration" not in err_lower
 
 
 def test_status(acfactory):