test: guard import smoke test against vacuous pass on empty walk

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_imports.py

@@ -23,6 +23,8 @@ def test_all_submodules_import():
         )
     ]
 
+    assert module_names, "walk_packages found no submodules - check smileyface.__path__"
+
     for name in module_names:
         try:
             importlib.import_module(name)