Fix/multi forecast fut cov

[dumjax]

Fixes when only fut cov.

[codecov-commenter]

Codecov Report

Base: 93.93% // Head: 93.91% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (5c9514b) compared to base (f0035e3).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1481      +/-   ##
==========================================
- Coverage   93.93%   93.91%   -0.03%     
==========================================
  Files         122      122              
  Lines       10742    10728      -14     
==========================================
- Hits        10091    10075      -16     
- Misses        651      653       +2     

Impacted Files	Coverage Δ
darts/models/forecasting/forecasting_model.py	97.05% <100.00%> (ø)
darts/timeseries.py	91.78% <0.00%> (-0.23%)	⬇️
darts/ad/anomaly_model/filtering_am.py	91.93% <0.00%> (-0.13%)	⬇️
...arts/models/forecasting/torch_forecasting_model.py	89.52% <0.00%> (-0.05%)	⬇️
darts/models/forecasting/block_rnn_model.py	98.24% <0.00%> (-0.04%)	⬇️
darts/models/forecasting/nhits.py	99.27% <0.00%> (-0.01%)	⬇️
darts/datasets/__init__.py	100.00% <0.00%> (ø)

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[hrzn]

Fixes #1466

[hrzn]

Thanks!