json: Don't panic for nil Encode{Time, Duration}

[abhinav]

Fixes #834

The JSON encoder assumes that encoders for time.Time and
time.Duration are always specified, which causes nil pointer
dereference panics.

Fix this by treating nil encoders for time and duration as no-ops. This
will fall back to existing logic in the JSON encoder that handles no-op
time and duration encoders.

[codecov]

Codecov Report

Merging #835 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #835   +/-   ##
=======================================
  Coverage   98.33%   98.33%           
=======================================
  Files          43       43           
  Lines        2347     2349    +2     
=======================================
+ Hits         2308     2310    +2     
  Misses         32       32           
  Partials        7        7           

Impacted Files	Coverage Δ
zapcore/json_encoder.go	100.00% <100.00%> (ø)

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[prashantv]

This looks related to #791 where we chose to error out on creation if the time key is set, but there was no time encoder.

I think the behavioue we'll end up with is:

• If the time key is specified, a time encoder must be specified.
• If no time key is specified, and there's no time encoder, then times will be encoded with a default

Should we have the same default behaviour even when a time key is set?

[abhinav]

Before we change the default behavior, I want to clarify that we have
two API contracts under consideration here.

• For encoders registered with zap.RegisterEncoder, the expectation
  since EncoderConfig and AtomicLevel validation added #791 is that zap.Config.Build will not invoke the constructor
  with a nil EncodeTime if TimeKey is not set.
• The breakage fixed in this change is a bug in
  zapcore.NewJSONEncoder: it did not handle a nil EncodeTime or
  EncodeDuration regardless of the value of TimeKey.

We can change Config.Build's behavior to set EncodeTime to a non-
nil value when TimeKey is set, and to set EncodeDuration to a non-
nil value if unset. However, this default would apply to all encoders,
including custom encoders. Are we sure we want this to be the default
behavior for all encoders?

[prashantv]

That's a good point, seems reasonable that different encoders may have different defaults.