Remove nil == nil if-statement

[rwelin]

Found using the nilness analysis:

$ go install golang.org/x/tools/go/analysis/passes/nilness/cmd/nilness
$ go vet -vettool=$(which nilness)
\# github.com/json-iterator/go
./iter_object.go:241:10: tautological condition: nil == nil

[codecov]

Codecov Report

Merging #420 into master will increase coverage by 0.05%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #420      +/-   ##
==========================================
+ Coverage   86.46%   86.52%   +0.05%     
==========================================
  Files          41       41              
  Lines        5105     5104       -1     
==========================================
+ Hits         4414     4416       +2     
+ Misses        555      552       -3     
  Partials      136      136

Impacted Files	Coverage Δ
iter_object.go	68.25% <0%> (+0.35%)	⬆️
reflect_struct_decoder.go	81.62% <0%> (+0.26%)	⬆️

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[rwelin]

Is it correct that readObjectFieldAsBytes is actually unused and could be completely removed instead?