Fix making backtest if all segments start with NaNs

[Mr-Geekman]

IMPORTANT: Please do not create a Pull Request without creating an issue first.

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?

Type of Change

• Examples / docs / tutorials / contributors update
• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Proposed Changes

Look #725.

Related Issue

#725.

Closing issues

Closes #725.

[github-actions]

🚀 Deployed on https://deploy-preview-728--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #728 (e7f0482) into master (d85c7f0) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #728   +/-   ##
=======================================
  Coverage   83.67%   83.67%           
=======================================
  Files         120      120           
  Lines        6629     6629           
=======================================
  Hits         5547     5547           
  Misses       1082     1082           

Impacted Files	Coverage Δ
etna/pipeline/base.py	94.44% <100.00%> (ø)

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more

[alex-hse-repository]

May be we also need test that run the backtest on dataset with nans in the begging and FoldMask logic

[alex-hse-repository]

👍