IndexFeatureFile: fix a crash on VCFs with 0 records

[droazen]

Resolves #4269

[droazen]

@lbergelson please review

[lbergelson]

Yes, do this.

[codecov-io]

Codecov Report

Merging #5795 into master will decrease coverage by 6.688%.
The diff coverage is 10%.

@@               Coverage Diff               @@
##              master     #5795       +/-   ##
===============================================
- Coverage     87.002%   80.314%   -6.688%     
+ Complexity     32103     30482     -1621     
===============================================
  Files           1974      1974               
  Lines         147214    147223        +9     
  Branches       16215     16216        +1     
===============================================
- Hits          128079    118240     -9839     
- Misses         13231     23280    +10049     
+ Partials        5904      5703      -201

Impacted Files	Coverage Δ	Complexity Δ
...lbender/tools/IndexFeatureFileIntegrationTest.java	0.442% <0%> (-90.802%)	1 <0> (-31)
...utils/codecs/ProgressReportingDelegatingCodec.java	43.137% <100%> (ø)	12 <0> (+1)	⬆️
...kers/filters/VariantFiltrationIntegrationTest.java	0.826% <0%> (-99.174%)	1% <0%> (-25%)
...dorientation/CollectF1R2CountsIntegrationTest.java	0.917% <0%> (-99.083%)	1% <0%> (-12%)
.../walkers/bqsr/BaseRecalibratorIntegrationTest.java	1.031% <0%> (-98.969%)	1% <0%> (-7%)
...ers/vqsr/FilterVariantTranchesIntegrationTest.java	1.053% <0%> (-98.947%)	1% <0%> (-5%)
...s/variantutils/VariantsToTableIntegrationTest.java	1.205% <0%> (-98.795%)	1% <0%> (-20%)
...tion/LearnReadOrientationModelIntegrationTest.java	1.724% <0%> (-98.276%)	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%)
... and 154 more