Restore support for old binary XGBoost models

[hcho3]

XGBoost models from 2 years ago (1.5 or older) won't have num_target field in the model. The model parser shouldn't fail if num_target is 0.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (ffa3fdb) 84.25% compared to head (bcdf694) 84.35%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## mainline #547 +/- ## ============================================ + Coverage 84.25% 84.35% +0.10% ============================================ Files 70 70 Lines 6064 6066 +2 Branches 510 510 ============================================ + Hits 5109 5117 +8 + Misses 955 949 -6 ```

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