Cleanup build warnings

[lintool]

hey @stephaniewhoo can you take a look? some of this is your code, I think.

Map<String, Object> json = new HashMap();

Just suppressing may not be the cleanest collection... is there a more appropriate specialized type than Object?

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Otherwise, fixed a bunch of minor build warnings.

[codecov-commenter]

Codecov Report

Merging #1710 (dea9498) into master (349db4c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #1710   +/-   ##
=========================================
  Coverage     57.58%   57.58%           
  Complexity     1032     1032           
=========================================
  Files           173      173           
  Lines          9781     9781           
  Branches       1344     1344           
=========================================
  Hits           5632     5632           
  Misses         3675     3675           
  Partials        474      474           

Impacted Files	Coverage Δ
.../anserini/collection/CommonCrawlWetCollection.java	74.35% <ø> (ø)
...ava/io/anserini/collection/DocumentCollection.java	57.33% <ø> (ø)
...o/anserini/collection/FeverSentenceCollection.java	83.63% <ø> (ø)
.../java/io/anserini/collection/WarcBaseDocument.java	42.68% <ø> (ø)
...in/java/io/anserini/ltr/FeatureExtractorUtils.java	66.26% <ø> (ø)
...c/main/java/io/anserini/ltr/QueryFieldContext.java	59.37% <ø> (ø)
...anserini/search/SimpleNearestNeighborSearcher.java	89.74% <ø> (ø)
...arch/topicreader/BackgroundLinkingTopicReader.java	29.76% <ø> (ø)
...in/java/io/anserini/search/topicreader/Topics.java	100.00% <100.00%> (ø)

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[stephaniewhoo]

The Object may be String or List<String>, so I am afraid that it cannot be further specified.

[lintool]

Okay, in that case can you do a CR and give me a 🚢 it of it looks good?

[stephaniewhoo]

Lgtm