XGBoost: Do not panic if a tree split is degenerate

[Dig-Doug]

Fixes #363.

Checklist

• My branch is up-to-date with development branch.
• Everything works and tested on latest stable Rust.
• Coverage and Linting have been applied

Current behaviour

XGRegressor can panic.

New expected behaviour

Does not panic.

[Mec-iS]

thank you. taking a look

[codecov]

Codecov Report

❌ Patch coverage is 21.42857% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.60%. Comparing base (70d8a0f) to head (8b24cc7).
⚠️ Report is 13 commits behind head on development.

Files with missing lines	Patch %	Lines
src/xgboost/xgb_regressor.rs	21.42%	22 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #364      +/-   ##
===============================================
- Coverage        45.59%   44.60%   -0.99%     
===============================================
  Files               93       95       +2     
  Lines             8034     8021      -13     
===============================================
- Hits              3663     3578      -85     
- Misses            4371     4443      +72     

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[Mec-iS]

it would be nice if it is possible to have a test to cover the case.