Fix returning types of evaluators

[maypink]

Fixes for #221 issue -- the bug was not in adaptive mutations, but in specified types of evaluation functions. It allowed evaluators to return Optional[GraphEvalResult] eval result, which is not fine for further optimization

[codecov-commenter]

Codecov Report

Merging #223 (d0dcb98) into main (b6cce63) will decrease coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #223      +/-   ##
==========================================
- Coverage   73.83%   73.82%   -0.01%     
==========================================
  Files         131      131              
  Lines        7842     7841       -1     
==========================================
- Hits         5790     5789       -1     
  Misses       2052     2052              

Files	Coverage Δ
golem/core/optimisers/genetic/evaluation.py	94.37% <100.00%> (-0.04%)	:arrow_down:
golem/core/optimisers/meta/surrogate_evaluator.py	96.00% <100.00%> (ø)

... and 4 files with indirect coverage changes