NPE in force merge when non knn doc gets updated to knn doc across segments

[vamshin]

Issue #, if available:
#210

Description of changes:
NPE occurs forcemerge when non knn doc gets updated to knn doc. This happens when a a doc in non knn in one segment and gets updated to knn doc in another segment and forcemerge occurs.

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[codecov]

Codecov Report

Merging #212 into master will increase coverage by 0.39%.
The diff coverage is 75.00%.

@@             Coverage Diff              @@
##             master     #212      +/-   ##
============================================
+ Coverage     76.95%   77.34%   +0.39%     
- Complexity      302      307       +5     
============================================
  Files            50       50              
  Lines          1189     1192       +3     
  Branches        100      101       +1     
============================================
+ Hits            915      922       +7     
+ Misses          225      221       -4     
  Partials         49       49              

Impacted Files	Coverage Δ	Complexity Δ
...n/index/codec/KNN80Codec/KNN80DocValuesReader.java	75.00% <75.00%> (+5.76%)	5.00 <0.00> (+2.00)
...relasticsearch/knn/index/KNNVectorFieldMapper.java	79.28% <0.00%> (+1.42%)	17.00% <0.00%> (+2.00%)
...index/codec/KNN80Codec/KNN80DocValuesConsumer.java	65.82% <0.00%> (+2.53%)	12.00% <0.00%> (+1.00%)

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[jmazanec15]

Thanks for the fix!