fix npe when has null value tag field in pojo

[flashmouse]

use addFieldsFromPOJO method build point will cause npe when some tag field hasn't set any value.

[codecov-io]

Codecov Report

Merging #598 into master will decrease coverage by 0.03%. The diff coverage is 50%.

@@             Coverage Diff              @@
##             master     #598      +/-   ##
============================================
- Coverage     88.19%   88.16%   -0.04%     
  Complexity      709      709              
============================================
  Files            69       69              
  Lines          2449     2450       +1     
  Branches        252      253       +1     
============================================
  Hits           2160     2160              
  Misses          204      204              
- Partials         85       86       +1

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/org/influxdb/dto/Point.java	91.5% <50%> (-0.46%)	41 <0> (ø)

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