skip timestamp if EncodeTime is nil (#645)

[errpunk]

This PR fix the issue #645
Program will panic when user set the TimeKey and did not set EncodeTime explicitly.
I've noticed that in consoleEncoder it will skip timestamp if EncodeTime is nil, so I add this into jsonEncoder to ensure behavioral consistency.

[codecov]

Codecov Report

Merging #646 into master will decrease coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #646     +/-   ##
=========================================
- Coverage   97.38%   97.28%   -0.1%     
=========================================
  Files          40       40             
  Lines        2102     2102             
=========================================
- Hits         2047     2045      -2     
- Misses         47       48      +1     
- Partials        8        9      +1

Impacted Files	Coverage Δ
zapcore/json_encoder.go	100% <100%> (ø)	⬆️
zapcore/sampler.go	95.83% <0%> (-4.17%)	⬇️

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[abhinav]

Superseded by #791