JSFiddle - React, Tailwind, and code Playground

JavaScript

var test = {  
   "person":{  
      "id":12345,
      "name":"John Doe",
      "phones":{  
         "home":"800-123-4567",
         "mobile":"null"
      },
      "email":[  
         "[email protected]",
         "[email protected]"
      ],
      "dateOfBirth":"null",
      "registered":true,
      "emergencyContacts":[  
         {  
            "name":"Jane Doe",
            "phone":"null",
            "relationship":"spouse"
         },
         {  
            "name":"Justin Doe",
            "phone":"877-123-1212",
            "relationship":"null"
         }
      ]
   }
};

console.log(test);

function isNull(obj, key) {
	return (obj[key] == null || obj[key] === undefined || obj[key] === "null");
}

function validate(obj) {
	var objKeys = Object.keys(obj);
  objKeys.forEach((key) => {
  	if(isNull(obj, key)) {
    	obj[key] = "";
    }
    if(typeof(obj[key]) == "object") {
    	validate(obj[key]);
    }
  });
}

validate(test);
console.log(test);