Set predicted recall to zero for docs without predicted labels

[cbartz]

It is currently (v.0.1.0) possible for a document with null predicted labels to receive a non-zero recall prediction from Qualle. By definition, for a document with null predicted labels, the recall can only be null. This PR fixes this by bypassing the predictor and directly nulling the recall for documents without predicted labels.

[codecov-commenter]

Codecov Report

Base: 93.43% // Head: 93.64% // Increases project coverage by +0.21% 🎉

Coverage data is based on head (86c7690) compared to base (ed521ea).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
+ Coverage   93.43%   93.64%   +0.21%     
==========================================
  Files          20       20              
  Lines         685      708      +23     
==========================================
+ Hits          640      663      +23     
  Misses         45       45              

Impacted Files	Coverage Δ
qualle/evaluate.py	100.00% <ø> (ø)
qualle/features/base.py	100.00% <ø> (ø)
qualle/features/combined.py	100.00% <ø> (ø)
qualle/features/confidence.py	100.00% <ø> (ø)
qualle/features/label_calibration/base.py	100.00% <ø> (ø)
...ures/label_calibration/simple_label_calibration.py	100.00% <ø> (ø)
...es/label_calibration/thesauri_label_calibration.py	99.04% <ø> (ø)
qualle/features/text.py	100.00% <ø> (ø)
qualle/interface/cli.py	53.08% <ø> (ø)
qualle/interface/config.py	100.00% <ø> (ø)
... and 10 more

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