Fix NPE when getting the bound elements in <nuxeo-layout>

In <nuxeo-layout>, the _getBoundElements(property) method is used to get the elements of the layout that are bound to a provided property.
However, in it it is assumed that all the nodeInfo objects will contain a bindings array, meaning that they are bound to at least one property, which is not always the case (ex: if a layout uses a <label> element).
The goal is to check if there are indeed bindings before actually trying to iterate over them.