Odd - I set up the omni sensor with some text as the variable attribute and it was passed just fine through to node red. Are you using a device node? How are you configuring it?
Also there doesn't seem to be a way to set the omni sensor variable within HE. This particular post (and code) might do the trick however Virtual Omni Sensor variable doesn't seem to work as trigger (possible bug?)
EDIT: Just to check maker API have you set up any devices at all to test?
