Fix putting nil to fmt.Stringer interface

[fpuc]

No description provided.

[CLAassistant]

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[codecov]

Codecov Report

Merging #674 into master will decrease coverage by 0.09%.
The diff coverage is 50%.

@@            Coverage Diff            @@
##           master     #674     +/-   ##
=========================================
- Coverage   97.38%   97.29%   -0.1%     
=========================================
  Files          40       40             
  Lines        2102     2105      +3     
=========================================
+ Hits         2047     2048      +1     
- Misses         47       48      +1     
- Partials        8        9      +1

Impacted Files	Coverage Δ
zapcore/field.go	97.5% <50%> (-2.5%)	⬇️

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[fpuc]

closing in favour of #675