Assure the description is loaded as string (backport of #1107)

If this isn't explicitly specified, the description string might be interpreted as a yaml content, which leads to problems, obviously.

ur_robot_driver/launch/ur_control.launch.py

@@ -30,7 +30,7 @@
 # Author: Denis Stogl
 
 from launch_ros.actions import Node
-from launch_ros.parameter_descriptions import ParameterFile
+from launch_ros.parameter_descriptions import ParameterFile, ParameterValue
 from launch_ros.substitutions import FindPackageShare
 
 from launch import LaunchDescription
@@ -200,7 +200,9 @@ def launch_setup(context, *args, **kwargs):
             " ",
         ]
     )
-    robot_description = {"robot_description": robot_description_content}
+    robot_description = {
+        "robot_description": ParameterValue(value=robot_description_content, value_type=str)
+    }
 
     initial_joint_controllers = PathJoinSubstitution(
         [FindPackageShare(runtime_config_package), "config", controllers_file]