fixed a bug in CalculateContamination when contamination is zero

[davidbenjamin]

@takutosato This fixes a bug that Scott Carter found.

[codecov]

Codecov Report

Merging #5971 into master will decrease coverage by 6.777%.
The diff coverage is 100%.

@@               Coverage Diff               @@
##              master     #5971       +/-   ##
===============================================
- Coverage     86.929%   80.152%   -6.777%     
+ Complexity     32714     31026     -1688     
===============================================
  Files           2013      2013               
  Lines         151268    151268               
  Branches       16604     16604               
===============================================
- Hits          131496    121245    -10251     
- Misses         13718     24175    +10457     
+ Partials        6054      5848      -206

Impacted Files	Coverage Δ	Complexity Δ
...ools/walkers/contamination/ContaminationModel.java	87.5% <100%> (ø)	39 <0> (ø)	⬇️
...dorientation/CollectF1R2CountsIntegrationTest.java	0.714% <0%> (-99.286%)	1% <0%> (-14%)
...kers/filters/VariantFiltrationIntegrationTest.java	0.826% <0%> (-99.174%)	1% <0%> (-25%)
.../walkers/bqsr/BaseRecalibratorIntegrationTest.java	1.031% <0%> (-98.969%)	1% <0%> (-7%)
...s/variantutils/VariantsToTableIntegrationTest.java	1.042% <0%> (-98.958%)	1% <0%> (-21%)
...ers/vqsr/FilterVariantTranchesIntegrationTest.java	1.053% <0%> (-98.947%)	1% <0%> (-5%)
...on/FindBreakpointEvidenceSparkIntegrationTest.java	1.754% <0%> (-98.246%)	1% <0%> (-6%)
...bender/tools/spark/PileupSparkIntegrationTest.java	2.041% <0%> (-97.959%)	2% <0%> (-13%)
...tute/hellbender/tools/FlagStatIntegrationTest.java	2.083% <0%> (-97.917%)	1% <0%> (-5%)
...rs/variantutils/SelectVariantsIntegrationTest.java	0.25% <0%> (-97.75%)	1% <0%> (-70%)
... and 157 more