Update empty field to contain value - rest API - sharepoint-2013

Question

So I have a snippet of code that will update a field value if the field has content, although if the field that I'm trying to update is null than the value won't update. Am I doing something wrong?
siteURL = _spPageContextInfo.webAbsoluteUrl;
var apiPath = _spPageContextInfo.webAbsoluteUrl +"/_api/lists/getbytitle('Training%20Copy')/items/getbyid(9)";
$.ajax({
url: apiPath,
type: "POST",
headers: {
Accept: "application/json;odata=verbose"
},
data: JSON.stringify
({
__metadata:
{
type: "SP.Data.Training_x0020_CopyItem"
},
Admin_x0020_Function: "Have content"
}),
headers: {
"Accept": "application/json;odata=verbose",
"Content-Type": "application/json;odata=verbose",
"X-RequestDigest": $("#__REQUESTDIGEST").val(),
"IF-MATCH": "*",
"X-HTTP-Method": "MERGE"
},
async: false, success: function(data) {
console.log("Item updated successfully");
}, eror: function(data) {
console.log("An error occurred. Please try again.");
}
})

Answer 1

There are two headers parameters in your ajax request, it is not clear whether it will affect.
My test code for your reference:
<script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-1.9.1.min.js" type="text/javascript"></script>
<script>
$(document).ready(function () {
Update()
function Update(){
$.ajax({
url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/GetByTitle('Doc')/items(9)",
type: "POST",
headers: {
"accept": "application/json;odata=verbose",
"X-RequestDigest": $("#__REQUESTDIGEST").val(),
"content-Type": "application/json;odata=verbose",
"IF-MATCH": "*",
"X-HTTP-Method": "MERGE"
},
data: JSON.stringify({__metadata:{'type':'SP.Data.DocItem'},test:'test'}),
/*where Title is column name and add your desired new data*/
success: function(data) {
console.log(data);
},
error: function(error) {
alert(JSON.stringify(error));
}
});
}
})
</script>