Get rid of unnecessary checks in composable_node_container. (#364)

valid_composable_nodes is a local variable that we know
is a list.  So we can just do the idiomatic Python thing
and check with a simple "if" statement.  This is also
faster.

Signed-off-by: Chris Lalancette <[email protected]>

Author: clalancette

launch_ros/launch_ros/actions/composable_node_container.py

@@ -85,10 +85,7 @@ def execute(self, context: LaunchContext) -> Optional[List[Action]]:
                 if node_object.condition() is None or node_object.condition().evaluate(context):
                     valid_composable_nodes.append(node_object)
 
-        if (
-            valid_composable_nodes is not None and
-            len(valid_composable_nodes) > 0
-        ):
+        if valid_composable_nodes:
             from .load_composable_nodes import LoadComposableNodes
             # Perform load action once the container has started.
             load_actions = [