Fix missing direction of ray (#3297)

[ffgao]

🦟 Bug fix

Fixes #3297

Summary

When 'relativeEndPos' and 'relativeStartPos' are close, zero vector will be generated, which will cause the ray to lose its direction forever.

void RayShape::SetLength(double _len)
{
  this->contactLen = _len;

  ignition::math::Vector3d dir = this->relativeEndPos - this->relativeStartPos;
  dir.Normalize();

  this->relativeEndPos = dir * _len + this->relativeStartPos;
}

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
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• All tests passed (See test coverage)
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[ffgao]

Expected behavior: