Support raw empty objects

[cmaster11]

There is a peculiar Joi detail where Joi.object() and Joi.object({}) behave in different ways:

• Joi.object() allows by default any key/value pair.
• Joi.object({}) DOES NOT allow by default any key/value pair.

When generating TS definitions, Joi.object() is perfectly represented by object, but Joi.object({}) should be a forcefully empty object {}.

This PR addresses this last situation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (12420fe) 98.77% compared to head (7501e89) 98.79%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #409      +/-   ##
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+ Coverage   98.77%   98.79%   +0.01%     
==========================================
  Files           9        9              
  Lines         573      580       +7     
  Branches      218      224       +6     
==========================================
+ Hits          566      573       +7     
  Misses          7        7              

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