Fixed #34903, Refs #34825 -- Made workers initialization respect empty set of used connections.

Thanks to David Smith for the investigation & patch.

Regression in 2128a73713735fb794ca6565fd5d7792293f5cfa.
Follow up to a5905b164dbf52e59fa646af9c3d523c0804d86a.

Co-authored-by: David Sanders <shang.xiao.sanders@gmail.com>

django/test/runner.py

@@ -431,7 +431,7 @@ def _init_worker(
         django.setup()
         setup_test_environment(debug=debug_mode)
 
-    db_aliases = used_aliases or connections
+    db_aliases = used_aliases if used_aliases is not None else connections
     for alias in db_aliases:
         connection = connections[alias]
         if start_method == "spawn":