SimpleTree: Fix (some?) nan predictions

[janezd]

Issue

Fixes #6001, which results from SimpleTree predicting nan when all target values in a branch are nan.

Description of changes

Add the necessary checks. There may be other cases that I have missed (the code is long), but I don't want to add a blanket check for nans at the end of predictor because it could hide problems in tree inducer..

Includes

• [X] Code changes
• [X] Tests

[codecov[bot]]

Codecov Report

Merging #6038 (b8d7c8f) into master (6d95493) will decrease coverage by 0.00%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6038      +/-   ##
==========================================
- Coverage   86.60%   86.59%   -0.01%     
==========================================
  Files         315      315              
  Lines       67317    67317              
==========================================
- Hits        58297    58296       -1     
- Misses       9020     9021       +1