UnhandledPromiseRejectionWarning when date is left blank

I'm currently creating a website using MEAN stack, and I'm almost done, but there's a problem with the "user editing his/her profile" part. When the user edits anything ALONG WITH editing his/her birthdate, the edit works just fine and all is updated in the database, however when the birthdate field is left empty I get the following error in node:

UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): ValidationError: CastError: Cast to Date failed for value "Invalid Date" at path "birthdate"

Here's my nodejs code:

exports.editInformation = function(req, res) {
        var id = req.params.userID;
        var body = req.body;

        User.findOne({_id:id}, function(err, user) {
            if(err)  
                res.send("error");
            else {
                if(!user) 
                    res.send("user not found");
                else {    
                    if(body.name) 
                        user.name = body.name;

                    if(body.birthdate)
                        user.birthdate = new Date(body.birthdate);

                    if(typeof body.phone != "undefined" && body.phone.length > 0) 
                        user.phone = body.phone;

                    if(typeof body.gender != "undefined" && body.gender.length > 0) 
                        user.gender = body.gender;

                    if(typeof body.address != "undefined" && body.address.length > 0) 
                        user.address = body.address;

                    if(typeof body.email != "undefined" && body.email.length > 0) 
                        user.email = body.email;

                    if(typeof file != "undefined") 
                        user.profilePic = file.filename;

                    user.save();
                }
            }
        });
}

via thelili700