Fix marker creation: allow zero scale for geometric shapes

[rhaschke]

• Link geometries like boxes, spheres, and cylinders might explicitly have a zero size. Don't reset their scale to (1,1,1)!
  Instead initialize a correct scaling by other means.
• As zero-size markers will be invisible, drop them from the generated marker arrays.

[codecov]

Codecov Report

Base: 58.85% // Head: 58.85% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (f45e355) compared to base (eae0bdc).
Patch coverage: 20.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #430      +/-   ##
==========================================
- Coverage   58.85%   58.85%   -0.00%     
==========================================
  Files          89       89              
  Lines        8386     8387       +1     
==========================================
  Hits         4935     4935              
- Misses       3451     3452       +1     

Impacted Files	Coverage Δ
rviz_marker_tools/src/marker_creation.cpp	29.37% <0.00%> (-0.27%)	⬇️
core/src/marker_tools.cpp	68.19% <50.00%> (-0.42%)	⬇️

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