That's what I figured, you print it inline, and the asynchronous request most likely hasn't returned any data when you try printing the model data 
Use the following:
var oModel = new sap.ui.model.odata.ODataModel("http://services.odata.org/V3/Northwind/Northwind.svc/");
sap.ui.getCore().setModel(oModel);
oModel.attachRequestCompleted(function() {
console.log(sap.ui.getCore().getModel().getProperty("/Customers"));
})