fix empty biz body unwrapBinary EOF error

[wodeqiangne]

What type of PR is this?

fix: A bug fix, UnwrapBinaryMessage return an EOF error when the biz body is empty

(Optional) Translate the PR title into Chinese.

修复空业务body在unwrapBinaryMessage遇到的EOF错误

(Optional) Which issue(s) this PR fixes:

Fixes #69

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