[JS][Flow] Fix body serialization when body is falsy

Created by: null-dev

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

This fixes serialization of the request body in the javascript-flowtyped generator when the request body is falsy. Before this fix, some request bodies consisting of raw boolean/integer literals would be incorrectly serialized as "{}" (e.g. false is serialized as "{}").

@CodeNinjai (2017/07) @frol (2017/07) @cliffano (2017/07)