Fix handling of nil empty interface

[toffaletti]

Before:

go test .
--- FAIL: Test_omitempty_nil_interface (0.00s)
        Error Trace:    jsoniter_interface_test.go:371
	Error:      	Not equal: 
	            	expected: "{}"
	            	actual: "{\"field\":null}"
FAIL
FAIL	github.com/json-iterator/go	0.148s

Relevant code in reflect package: https://golang.org/src/reflect/value.go#L132

[codecov]

Codecov Report

Merging #169 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
- Coverage   84.74%   84.73%   -0.02%     
==========================================
  Files          41       41              
  Lines        5199     5200       +1     
==========================================
  Hits         4406     4406              
- Misses        632      633       +1     
  Partials      161      161

Impacted Files	Coverage Δ
feature_reflect_native.go	64.81% <100%> (+0.62%)	⬆️
feature_iter_string.go	89.51% <0%> (-2.1%)	⬇️

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