handle nil body for JSON binding

[mllu]

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In the real world, it probably shouldn't matter. It just happened when I forgot to pass body to http.NewRequest and the function Bind panicked under that situation.

If we always assume body will not be nil, please let me know, I will close this PR. Thank you.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@521d06c). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1638   +/-   ##
=========================================
  Coverage          ?   99.29%           
=========================================
  Files             ?       40           
  Lines             ?     1973           
  Branches          ?        0           
=========================================
  Hits              ?     1959           
  Misses            ?       10           
  Partials          ?        4

Impacted Files	Coverage Δ
binding/json.go	100% <100%> (ø)

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[appleboy]

@thinkerou need your review.