msa: fix segfault

[v4hn]

happened when a group was defined in the launch file without a ns attribute.

[codecov]

Codecov Report

Merging #2564 (9a5c4e6) into master (460c152) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

❗ Current head 9a5c4e6 differs from pull request most recent head 5dc271d. Consider uploading reports for the commit 5dc271d to get more accurate results

@@            Coverage Diff             @@
##           master    #2564      +/-   ##
==========================================
- Coverage   60.21%   60.14%   -0.06%     
==========================================
  Files         351      351              
  Lines       26575    26576       +1     
==========================================
- Hits        16000    15982      -18     
- Misses      10575    10594      +19     

Impacted Files	Coverage Δ
...t_setup_assistant/src/tools/moveit_config_data.cpp	19.19% <0.00%> (-0.02%)	⬇️
.../ompl_interface/src/detail/constrained_sampler.cpp	42.86% <0.00%> (-17.14%)	⬇️
moveit_core/robot_model/src/joint_model_group.cpp	63.08% <0.00%> (-2.46%)	⬇️
...e/src/parameterization/model_based_state_space.cpp	70.40% <0.00%> (-2.39%)	⬇️
...on/pick_place/src/approach_and_translate_stage.cpp	72.14% <0.00%> (-0.81%)	⬇️
...meterization/work_space/pose_model_state_space.cpp	84.56% <0.00%> (-0.73%)	⬇️
...dl_kinematics_plugin/src/kdl_kinematics_plugin.cpp	76.11% <0.00%> (-0.44%)	⬇️
...nning_scene_monitor/src/planning_scene_monitor.cpp	67.62% <0.00%> (+0.16%)	⬆️
...ipulation/pick_place/src/manipulation_pipeline.cpp	73.41% <0.00%> (+1.07%)	⬆️

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[felixvd]

Should be backported as far as I can see.