Serialize Annotation object with all required fields even empty

[sathish-kumar-subramani]

Having JsonInclude:NON_EMPTY removes empty fields (e.g., "")
When the required fields are removed, API calls fail due to missing
fields in the request.

Let's use less restrictive JsonInclude:NON_ABSENT annotation, which
removes only null & absent fields. Required fields are anyway non-nullable.

The failing test commit 5a1724f shows how NON_EMPTY rule works

Failed tests: 
  AnnotationTest.serializesWithEmptyFields:80 
Expected: is "{\"path\":\"\",\"annotation_level\":\"notice\",\"message\":\"\",\"title\":\"\",\"start_line\":1,\"end_line\":2}"
     but: was "{\"annotation_level\":\"notice\",\"title\":\"\",\"start_line\":1,\"end_line\":2}"

^ from failed build logs

[codecov]

Codecov Report

Merging #110 (4543f28) into master (d0c05a4) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #110   +/-   ##
=========================================
  Coverage     73.06%   73.06%           
  Complexity      245      245           
=========================================
  Files            39       39           
  Lines           880      880           
  Branches         39       39           
=========================================
  Hits            643      643           
  Misses          212      212           
  Partials         25       25           

Impacted Files	Coverage Δ
.../java/com/spotify/github/v3/checks/Annotation.java	100.00% <ø> (ø)

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