Simplify use of argument paused

... avoiding code duplication

franka_gazebo/launch/panda.launch

@@ -31,6 +31,7 @@
 
   <include file="$(find gazebo_ros)/launch/empty_world.launch" if="$(arg gazebo)">
     <arg name="world_name" value="$(arg world)"/>
+    <!-- Always start in paused mode, and only unpause when spawning the model -->
     <arg name="paused" value="true"/>
     <arg name="gui" value="$(eval not arg('headless'))"/>
     <arg name="use_sim_time" value="true"/>
@@ -49,23 +50,10 @@
     <rosparam file="$(find franka_gazebo)/config/franka_hw_sim.yaml" subst_value="true" />
     <rosparam file="$(find franka_gazebo)/config/sim_controllers.yaml" subst_value="true" />
 
-    <node name="$(arg arm_id)_model_spawner"
-          pkg="gazebo_ros"
-          type="spawn_model"
-          if="$(arg paused)"
+    <node name="model_spawner" pkg="gazebo_ros" type="spawn_model"
           args="-param robot_description -urdf -model $(arg arm_id)
-                $(arg initial_joint_positions)
-                ">
-    </node>
-    <node name="$(arg arm_id)_model_spawner"
-          pkg="gazebo_ros"
-          type="spawn_model"
-          unless="$(arg paused)"
-          args="-param robot_description -urdf -model $(arg arm_id) -unpause
-                $(arg initial_joint_positions)
-                ">
-    </node>
-
+                $(eval '' if arg('paused') else '-unpause')
+                $(arg initial_joint_positions)"/>
 
     <!-- Spawn required ROS controllers -->
     <node pkg="controller_manager"