receiving null custom field data from ZAFClient

recently we've found different behavior for our public vs private apps. after registering our app we create a new instance of our widget.

client.on('app.registered', function (appData) {
  return new SessionSidebar(client, appData);
})

our widget attempts to read settings:

if (!isNullOrEmpty(this._metadata.settings.UserUidField)) {
    uidFieldName = ('user.customField:' + this._metadata.settings.UserUidField).trim();
}

our is null or empty parameter needs to check for the presence of string value "null" instead of simply the null type. we've found that on public apps if a user field setting is empty we'll receive a "null" string, while on private apps we'll receive a null type instead. 

function isNullOrEmpty(param) {
    return !param || typeof param !== "string" || param === "null" || param.trim() === "";
}

has anyone seen this behavior before?