Fix ConfigSampler to handle trials without hash

[d-a-bunin]

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Proposed Changes

• Add .user_attrs.get("hash") in ConfigSampler._get_unfinished_hashes
• Add additional check on if len(unfinished_hash) == 0 copied from GridSampler

Closing issues

Closes #607

[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.47%. Comparing base (797cfe3) to head (fac9dbd).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
etna/auto/optuna/config_sampler.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #616      +/-   ##
==========================================
+ Coverage   90.42%   90.47%   +0.05%     
==========================================
  Files         259      259              
  Lines       18036    18041       +5     
==========================================
+ Hits        16309    16323      +14     
+ Misses       1727     1718       -9     

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