REST API in SharePoint 2013 does Not support null values for filtering on list item queries.
However you can combine CAML query with REST API to get the required data.
Please refer below code :
(Note : please change your query/URL):
function _rest_Post(rquest) {
return $.ajax({
method: rquest.method,
url: rquest.url,
contentType: rquest.contentType,
headers: rquest.header,
data: JSON.stringify(rquest.body),
});
}
function GetByCaml(serviceParams) {
var req = {
method: 'POST',
url: url + "/_api/web/lists/getByTitle('" + serviceParams.create.lName + "')/getitems",
header: {
"X-RequestDigest": $("#__REQUESTDIGEST").val(),
"accept": "application/json;odata=verbose",
"content-type": "application/json;odata=verbose"
},
body: serviceParams.create.body,
contentType: "application/json;odata=verbose",
};
return _rest_Post(req);
}
function getData(){
var queryViewXml = "<View><Query><Where><IsNull><FieldRef Name='Project_x0020_Manager'/></IsNull></Where></Query><ViewFields><FieldRef Name='Title'/></ViewFields></View>";
var params = {};
params.create= {};
params.create.lName = "MyList";
params.create.filter = "";
params.create.body = {
'query':{
'__metadata': { 'type': 'SP.CamlQuery' },
'ViewXml': queryViewXml
}
}
return GetByCaml(params);
};
getData().then(function(data){
//success handler
}, function(error){
//failure handler
});