Installs an empty plugin description xml file if bullet is not found

[progtologist]

Fair warning: This only fixes the install workspace, in the devel workspace if bullet was not found at compile you will still get errors from pluginlib.

Description

Added an "empty" pluginlib xml file to be installed in place of collision_detector_bullet_description.xml if bullet is not found at compile time.
Related PR: #1839

Checklist

• Required by CI: Code is auto formatted using clang-format
• Extend the tutorials / documentation reference
• Document API changes relevant to the user in the MIGRATION.md notes
• Create tests, which fail without this PR reference
• Include a screenshot if changing a GUI
• While waiting for someone to review your request, please help review another open pull request to support the maintainers

[welcome]

Thanks for helping in improving MoveIt and open source robotics!

[rhaschke]

I approve. Please consider the comment below.

[codecov-io]

Codecov Report

Merging #1898 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1898      +/-   ##
==========================================
+ Coverage   50.26%   50.27%   +<.01%     
==========================================
  Files         313      313              
  Lines       24624    24626       +2     
==========================================
+ Hits        12378    12381       +3     
+ Misses      12246    12245       -1

Impacted Files	Coverage Δ
...nning_scene_monitor/src/planning_scene_monitor.cpp	55.53% <0%> (+0.14%)	⬆️
...ccupancy_map_monitor/src/occupancy_map_monitor.cpp	30.61% <0%> (+0.47%)	⬆️
...ccupancy_map_monitor/src/occupancy_map_updater.cpp	9.09% <0%> (+4.32%)	⬆️

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[j-petit]

Looks good to me, thanks for fixing this!

[welcome]

Congrats on getting your first MoveIt pull request merged and improving open source robotics!