[python-package] Fix class Booster: feature_types = None

[Roffild]

  in get_dump
    if self.feature_types is None:
AttributeError: 'Booster' object has no attribute 'feature_types'

[codecov-io]

Codecov Report

Merging #6705 (344cb16) into master (a4101de) will increase coverage by 0.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master    #6705      +/-   ##
==========================================
+ Coverage   81.55%   81.56%   +0.01%     
==========================================
  Files          13       13              
  Lines        3719     3760      +41     
==========================================
+ Hits         3033     3067      +34     
- Misses        686      693       +7     

Impacted Files	Coverage Δ
python-package/xgboost/data.py	62.63% <77.77%> (+0.07%)	⬆️
python-package/xgboost/sklearn.py	90.54% <84.00%> (-0.16%)	⬇️
python-package/xgboost/dask.py	82.57% <90.54%> (-0.10%)	⬇️
python-package/xgboost/core.py	81.94% <90.69%> (+0.09%)	⬆️
python-package/xgboost/training.py	95.60% <100.00%> (+0.28%)	⬆️

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[trivialfis]

Thanks for the fix!