Fix segfault from raycast when no ray hit

[jslee02]

The raycast operation causes a segfault when there is no hit. This is because DART attempts to dereference a nullptr in converting the ray hit result. This PR fixes it by changing the logic to convert ray hit results only when ray hits are found.

---

Before creating a pull request

• Document new methods and classes
• Format new code files using clang-format

Before merging a pull request

• Set version target by selecting a milestone on the right side
• Summarize this change in CHANGELOG.md
• Add unit test(s) for this change

[codecov]

Codecov Report

Merging #1461 into master will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master    #1461   +/-   ##
=======================================
  Coverage   58.23%   58.23%           
=======================================
  Files         412      412           
  Lines       29923    29928    +5     
=======================================
+ Hits        17425    17429    +4     
- Misses      12498    12499    +1     

Impacted Files	Coverage Δ
dart/collision/bullet/BulletCollisionDetector.cpp	70.27% <66.66%> (+0.12%)	⬆️