Check if EncodeLevel is set to avoid nil pointer dereference

[aerosol]

Hello,

Apologies in advance, this is my first go PR.

This PR checks if EncodeLevel is set before calling it on final encoder clone. This prevents panic on nil pointer dereference in case it hasn't been supplied by the user. For an untrained eye, the stacktrace wasn't making it super clear that EncodeLevel must be provided along with LevelKey, so hopefully an explicit check hints at it for someone reading the source code.

I wasn't sure if maybe providing a sensible default here would've been better, but looking at the console logger, a similar check is made.

Thanks.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #1058 (c2a7c3a) into master (a1155d9) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1058   +/-   ##
=======================================
  Coverage   98.29%   98.29%           
=======================================
  Files          48       48           
  Lines        2111     2111           
=======================================
  Hits         2075     2075           
  Misses         28       28           
  Partials        8        8           

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

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a1155d9...c2a7c3a. Read the comment docs.

[abhinav]

Thanks for the fix!