Fix BinnedPrecisionRecallCurve when thresholds not specified

[DuYicong515]

What does this PR do?

Fixes #969

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• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #968 (f523204) into master (10b3a85) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #968   +/-   ##
=====================================
- Coverage      95%    95%   -0%     
=====================================
  Files         177    177           
  Lines        7509   7509           
=====================================
- Hits         7137   7136    -1     
- Misses        372    373    +1     

[stancld]

LGTM, thanks for fixing this :]