Declare the namespace launch argument in state publisher launch

[mmporong]

Issue: Fixes #1149. turtlebot3_state_publisher.launch.py cannot be launched
directly because it reads LaunchConfiguration('namespace') without declaring it or
giving it a default, so it only works when included from robot.launch.py.

Fix: Give namespace a default value and declare it, mirroring what
d81e95a already did for robot.launch.py. The two launch files are now consistent.

-    namespace = LaunchConfiguration('namespace')
+    namespace = LaunchConfiguration('namespace', default='')

     return LaunchDescription([
+        DeclareLaunchArgument(
+            'namespace',
+            default_value='',
+            description='Namespace for nodes'),
         DeclareLaunchArgument(
             'use_sim_time',

Verification: tested against ros-humble-turtlebot3-bringup 2.3.6 with
TURTLEBOT3_MODEL=burger.

check	before	after
ros2 launch turtlebot3_bringup turtlebot3_state_publisher.launch.py	launch configuration 'namespace' does not exist	starts, got segment base_footprint
same with namespace:=tb1	same error	got segment tb1/base_footprint
ros2 launch turtlebot3_bringup robot.launch.py	3 nodes start	3 nodes start (unchanged)
... --show-args	namespace not listed	namespace listed
flake8 --max-line-length 99	clean	clean

Multi-robot behaviour is unchanged: omitting the argument keeps the namespace empty,
which is what robot.launch.py passes today.

The file is byte-identical on main, jazzy and humble, so the same patch applies
to the other branches if a backport is wanted.