Empty request body is not generated, but it should

[exoego]

Closes #195

The current implementation does not generate an empty request body (e.g., {} for application/javascript).
This leads to always-required top-level properties reported in #195

 PostUsersRequest: 
   type: object 
   properties: 
     name: 
       type: string 
   required:
     - name

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.09%. Comparing base (5c875d7) to head (57ee078).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #196   +/-   ##
=======================================
  Coverage   97.09%   97.09%           
=======================================
  Files          15       15           
  Lines         516      517    +1     
  Branches      125      125           
=======================================
+ Hits          501      502    +1     
  Misses         15       15           

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