Assume root namespace if not provided in ComposableNodeContainer (#186)

Fixes a regression introduced in #179

Fixes #185.

We apply the same logic in LifecycleNode to maintain backwards compatibility in Foxy.

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

launch_ros/launch_ros/actions/composable_node_container.py

@@ -77,6 +77,8 @@ def __init__(
                 )
             namespace = node_namespace
 
+        if not namespace:
+            namespace = '/'
         super().__init__(name=name, namespace=namespace, **kwargs)
         self.__composable_node_descriptions = composable_node_descriptions