Fix/filter static cov component

[hrzn]

TimeSeries.univariate_component() was broken when the series contained static covariates, this fixes it (and simplifies the method).

[dennisbader]

Thanks, looks good! Added a minor comment

[codecov-commenter]

Codecov Report

Merging #1128 (06a822f) into master (caccce1) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1128      +/-   ##
==========================================
+ Coverage   93.65%   93.75%   +0.09%     
==========================================
  Files          79       80       +1     
  Lines        8137     8144       +7     
==========================================
+ Hits         7621     7635      +14     
+ Misses        516      509       -7     

Impacted Files	Coverage Δ
darts/dataprocessing/dtw/dtw.py	94.20% <100.00%> (-0.13%)	⬇️
darts/models/components/transformer.py	100.00% <100.00%> (ø)
...arts/models/forecasting/torch_forecasting_model.py	89.54% <100.00%> (-0.05%)	⬇️
darts/models/forecasting/transformer_model.py	100.00% <100.00%> (ø)
darts/timeseries.py	92.23% <100.00%> (+0.01%)	⬆️
darts/models/forecasting/block_rnn_model.py	98.24% <0.00%> (-0.04%)	⬇️
darts/models/forecasting/nhits.py	98.55% <0.00%> (-0.02%)	⬇️
darts/datasets/__init__.py	100.00% <0.00%> (ø)
... and 2 more

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