ExtractDocumentLengths: prints out sum of doclengths, both lossy and lossless

[lintool]

Ref: osirrc/ciff#21

Adds check in ExtractDocumentLengths per above issue.

[codecov]

Codecov Report

Merging #1040 into master will increase coverage by 0.34%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1040      +/-   ##
============================================
+ Coverage     43.18%   43.53%   +0.34%     
- Complexity      610      622      +12     
============================================
  Files           128      128              
  Lines          7750     7759       +9     
  Branches       1131     1131              
============================================
+ Hits           3347     3378      +31     
+ Misses         4082     4062      -20     
+ Partials        321      319       -2     

Impacted Files	Coverage Δ	Complexity Δ
.../java/io/anserini/util/ExtractDocumentLengths.java	86.84% <100.00%> (+10.98%)	3.00 <0.00> (+1.00)
...anserini/ltr/feature/base/PMIFeatureExtractor.java	86.53% <0.00%> (+1.92%)	13.00% <0.00%> (+1.00%)
...java/io/anserini/ltr/feature/CountBigramPairs.java	89.61% <0.00%> (+24.67%)	33.00% <0.00%> (+10.00%)

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[chriskamphuis]

Code looks good, but does not work for core18.
core18 has documents with length 0 which will result in the index not storing a docvector and resulting the following line to return null, and consequently throwing an error.

Terms terms = reader.getTermVector(i, "contents");

[chriskamphuis]

(I suppose this problem already existed)

[lintool]

Fixed the issue you mentioned while I was at it...