Treat the default inventory_files as host list (ansible#4448)

The --inventory option to the ansible-playbook executable can accept a
value that is comma-separated. This comma-separated value is presumed to
be a host list and not a list of inventory files. This was the behavior
for the inventory_files variable before 4d295e8. There wasn't a reason
for its removal and it was an oversight made.

src/ansiblelint/runner.py

@@ -514,7 +514,7 @@ def _get_inventory_files(self, app: App) -> list[str]:
             # To avoid noisy warnings we pass localhost as current inventory:
             # [WARNING]: No inventory was parsed, only implicit localhost is available
             # [WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'
-            inventory_files = ["localhost"]
+            inventory_files = ["localhost,"]
 
         return inventory_files