handle null in json

[nicksnyder]

Treat null translations in JSON the same as being not present.

fixes #153

[codecov]

Codecov Report

Merging #166 into master will increase coverage by 1.44%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
+ Coverage   69.01%   70.46%   +1.44%     
==========================================
  Files          28       28              
  Lines        2353     2360       +7     
==========================================
+ Hits         1624     1663      +39     
+ Misses        639      609      -30     
+ Partials       90       88       -2

Impacted Files	Coverage Δ
v2/internal/message.go	75.43% <92.3%> (+31.51%)	⬆️

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