Fix TypeError when Docker returns null labels in container inspect

When a container has no labels, Docker returns {"Labels": null} in the
inspect response. The code was using .get("Labels", {}) which only
returns the default when the key is missing, not when it's null.

This caused _has_webterm_label() to raise TypeError, which was silently
caught by the event watcher's exception handler, causing it to reconnect
and miss the container start event.

Fixed by using .get("Labels") or {} pattern in:
- _handle_event() when processing start events
- _get_container_command() when extracting command label
- _get_container_theme() when extracting theme label

Added test for null labels case.

src/webterm/docker_watcher.py

@@ -151,7 +151,7 @@ class DockerWatcher:
 
         If label is 'auto', empty, or missing, returns default exec command.
         """
-        labels = container.get("Labels", {})
+        labels = container.get("Labels") or {}
         label_value = labels.get(LABEL_NAME)
 
         if _is_auto_label(label_value):
@@ -159,7 +159,7 @@ class DockerWatcher:
         return label_value or AUTO_COMMAND_SENTINEL
 
     def _get_container_theme(self, container: dict) -> str | None:
-        labels = container.get("Labels", {})
+        labels = container.get("Labels") or {}
         value = labels.get(THEME_LABEL)
         if isinstance(value, str) and value.strip():
             return value.strip()
@@ -287,12 +287,14 @@ class DockerWatcher:
             if status == 200:
                 container_info = json.loads(body)
                 # Convert to list format expected by _add_container
+                # Labels can be None if container has no labels
+                labels = container_info.get("Config", {}).get("Labels") or {}
                 container = {
                     "Id": container_id,
                     "Names": ["/" + container_info.get("Name", "").lstrip("/")],
-                    "Labels": container_info.get("Config", {}).get("Labels", {}),
+                    "Labels": labels,
                 }
-                if _has_webterm_label(container.get("Labels", {})):
+                if _has_webterm_label(labels):
                     await self._add_container(container)
         elif action == "die":
             await self._remove_container(container_id)

tests/test_docker_watcher.py

@@ -263,6 +263,36 @@ class TestDockerWatcherIntegration:
 
         session_manager.add_app.assert_called_once()
 
+    @pytest.mark.asyncio
+    async def test_handle_start_event_null_labels(self, session_manager):
+        """Container with null labels in Docker inspect response is handled gracefully."""
+        watcher = DockerWatcher(session_manager)
+
+        async def mock_request(method, path):
+            if "/containers/" in path and "/json" in path:
+                # Docker returns null for Labels when container has none
+                return (
+                    200,
+                    '{"Name": "/no-labels-container", "Config": {"Labels": null}}',
+                )
+            return 404, ""
+
+        watcher._docker_request = mock_request
+
+        event = {
+            "Action": "start",
+            "Actor": {
+                "ID": "container456",
+                "Attributes": {},
+            },
+        }
+
+        # Should not raise an exception
+        await watcher._handle_event(event)
+
+        # Should not add container (no webterm labels)
+        session_manager.add_app.assert_not_called()
+
 
 @pytest.mark.parametrize(
     ("labels", "expected"),