Improve handling of empty octomap messages and "clear octomap" actions

[FSund]

Description

Currently, if a PlanningScene message is published on the monitoried_planning_scene topic with an empty msg.world.octomap.octomap.data field, the octomap is skipped altogether in if it is a msg.is_diff is True (source). I think perhaps an empty data field is interpreted as the octomap field not being filled. But this makes it impossible to clear the octomap by sending an empty octomap message (for example via the "Clear octomap" button in the MotionPlanning panel in RViz).

This PR sets octomap.id to "empty" if the octomap is empty (this string can be anything except an empty string), and checks octomap.id.empty() instead of octomap.data.empty() to see if the octomap should be processed.

Fixes #3045.

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

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.12 🎉

Comparison is base (0f66b99) 61.60% compared to head (40bf88b) 61.72%.

❗ Current head 40bf88b differs from pull request most recent head 7ad5f24. Consider uploading reports for the commit 7ad5f24 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3134      +/-   ##
==========================================
+ Coverage   61.60%   61.72%   +0.12%     
==========================================
  Files         376      376              
  Lines       33309    33318       +9     
==========================================
+ Hits        20518    20562      +44     
+ Misses      12791    12756      -35     

Impacted Files	Coverage Δ
moveit_core/planning_scene/src/planning_scene.cpp	65.98% <100.00%> (+1.86%)	⬆️

... and 6 files with indirect coverage changes

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[rhaschke]

Thanks a lot for this fix and sorry that your PR was ignored for so long.
In prinicple I approve, but I suggest to set a non-empty id only if the octomap was actually cleared. Otherwise, a corresponding update will be always triggered.

[rhaschke]

Fixes #3380, #3342, #3045, #439