Replaced NULL with nullptr

[stephanie-eng]

Description

Replace NULL with nullptr throughout the project as per #841

[codecov]

Codecov Report

Merging #961 (7090f0a) into main (3092518) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #961      +/-   ##
==========================================
- Coverage   57.90%   57.90%   -0.00%     
==========================================
  Files         310      310              
  Lines       26113    26113              
==========================================
- Hits        15118    15117       -1     
- Misses      10995    10996       +1     

Impacted Files	Coverage Δ
...include/moveit/collision_detection/collision_env.h	100.00% <ø> (ø)
.../moveit/collision_detection_fcl/collision_common.h	89.19% <ø> (ø)
...raint_samplers/src/default_constraint_samplers.cpp	72.71% <0.00%> (ø)
...moveit/distance_field/propagation_distance_field.h	88.64% <ø> (ø)
...e_field/include/moveit/distance_field/voxel_grid.h	95.24% <ø> (ø)
...oveit/kinematic_constraints/kinematic_constraint.h	91.31% <ø> (ø)
...ene/include/moveit/planning_scene/planning_scene.h	50.00% <ø> (ø)
moveit_core/planning_scene/src/planning_scene.cpp	49.52% <0.00%> (ø)
...bot_model/include/moveit/robot_model/joint_model.h	81.95% <ø> (ø)
...del/include/moveit/robot_model/joint_model_group.h	87.18% <ø> (ø)
... and 7 more

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[henningkayser]

@stephanie-eng I just noticed that there are still plenty of "NULL" mentions in the comments. Shouldn't those be updated as well for consistency?