fix(xgboost): return error instead of panicking on empty training data

[SAY-5]

Fixes #446

Checklist

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

Current behaviour

XGRegressor::fit panics on a training set with zero rows. find_best_split runs 0..sorted_idxs.len() - 1, which underflows on an empty slice; in debug builds this is attempt to subtract with overflow and in release builds it surfaces as index out of bounds. A zero-row matrix is reachable through the public Array2::take, so this is reachable from safe user code.

New expected behaviour

fit validates that the training data has at least one row and returns Err(Failed::because(FailedError::ParametersError, ...)) for empty data, mirroring the existing subsample validation a few lines above. A model trained on no data is not useful, so this matches the error direction the reporter preferred. Non-empty inputs are unaffected. A regression test covers the empty-data case.