Clarify the behaviour of TimeSeriesImputerTransform in case of all NaN values

[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 #339.

Related Issue

#339.

Closing issues

Closes #339.

[codecov-commenter]

Codecov Report

Merging #427 (9d1ee74) into master (0e1f7c1) will decrease coverage by 34.06%.
The diff coverage is 55.22%.

@@             Coverage Diff             @@
##           master     #427       +/-   ##
===========================================
- Coverage   87.13%   53.07%   -34.07%     
===========================================
  Files          99      112       +13     
  Lines        5067     5276      +209     
===========================================
- Hits         4415     2800     -1615     
- Misses        652     2476     +1824     

Impacted Files	Coverage Δ
...na/transforms/decomposition/change_points_trend.py	40.95% <ø> (ø)
etna/transforms/decomposition/detrend.py	41.66% <ø> (ø)
etna/transforms/decomposition/stl.py	61.19% <ø> (ø)
etna/transforms/encoders/segment_encoder.py	42.10% <ø> (ø)
etna/transforms/feature_selection/base.py	45.00% <ø> (ø)
etna/transforms/feature_selection/filter.py	28.57% <ø> (ø)
etna/transforms/math/add_constant.py	61.90% <ø> (ø)
etna/transforms/math/lags.py	82.85% <ø> (ø)
etna/transforms/math/log.py	60.86% <ø> (ø)
etna/transforms/math/sklearn.py	29.90% <ø> (ø)
... and 97 more

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