# echo '{"empty inner hash": {}, "empty inner array": [], "some undef": null }' | ./tidyup_json 
null


# echo '{"empty inner hash": {}, "empty inner array": [1], "some undef": null }' | ./tidyup_json 
{
   "empty inner array" : [
      1
   ]
}


# echo '{"empty inner hash": {}, "empty inner array": [1], "some undef": 7 }' | ./tidyup_json 
{
   "empty inner array" : [
      1
   ],
   "some undef" : 7
}

# echo '{ }' | ./tidyup_json    ## !!! important to return null 
null