Disable use plotlypy setting when plotlypy is not available

[porink0424]

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This repository (optuna-dashboard) and Goptuna share common code.
This pull request may therefore be ported to Goptuna.
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Reference Issues/PRs

None

What does this implement/fix? Explain your changes.

Disable use plotlypy setting when plotlypy is not available.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.32%. Comparing base (cc0ab0f) to head (70c65ce).
Report is 152 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #948      +/-   ##
==========================================
+ Coverage   69.58%   70.32%   +0.74%     
==========================================
  Files          35       35              
  Lines        2377     2379       +2     
==========================================
+ Hits         1654     1673      +19     
+ Misses        723      706      -17     

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