Minor fixups in PlanningSceneDisplay

[rhaschke]

These commits fix some minor issues I noticed when looking into #2612. See individual commit messages for details.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@a3a4d8f). Click here to learn what that means.
The diff coverage is 60.00%.

@@            Coverage Diff            @@
##             master    #2618   +/-   ##
=========================================
  Coverage          ?   60.47%           
=========================================
  Files             ?      402           
  Lines             ?    29614           
  Branches          ?        0           
=========================================
  Hits              ?    17906           
  Misses            ?    11708           
  Partials          ?        0           

Impacted Files	Coverage Δ
...nclude/moveit/robot_state/cartesian_interpolator.h	85.72% <0.00%> (ø)
...al_motion_planner/src/trajectory_generator_ptp.cpp	98.94% <75.00%> (ø)

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[tylerjw]

This looks good. Thank you!