BaseStorage.set_trial_intermediate_valute to return None instead of bool.

[hvy]

Motivation

See #1309.

Description of the changes

Fixes #1309, in particular the following.

• The storage always sets. The check for "NOOP" is performed by the trial.

[codecov-commenter]

Codecov Report

Merging #1337 into master will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1337      +/-   ##
==========================================
- Coverage   87.83%   87.79%   -0.04%     
==========================================
  Files          96       96              
  Lines        7300     7301       +1     
==========================================
- Hits         6412     6410       -2     
- Misses        888      891       +3     

Impacted Files	Coverage Δ
optuna/storages/base.py	67.59% <ø> (ø)
optuna/storages/cached_storage.py	96.45% <100.00%> (-0.03%)	⬇️
optuna/storages/in_memory.py	97.69% <100.00%> (-0.05%)	⬇️
optuna/storages/rdb/storage.py	93.70% <100.00%> (-0.04%)	⬇️
optuna/storages/redis.py	94.18% <100.00%> (-0.06%)	⬇️
optuna/trial/_trial.py	91.77% <100.00%> (+0.15%)	⬆️
optuna/integration/skopt.py	90.83% <0.00%> (-1.10%)	⬇️
optuna/dashboard.py	60.64% <0.00%> (+0.38%)	⬆️

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[ytsmiling]

Thank you for addressing #1309. LGTM!

[HideakiImamura]

Thanks for the PR! LGTM.