Prevent NPEs in Feed beans

There are three bean classes provided by the plugin in the package org.onehippo.forge.feed.beans. Some of the getters return multivalue properties. But if for some reason the actual property is not found on the node the code in the beans produces a null pointer exception. This is an example:

public List<String> getCategory() {
       String[] catgries = getProperty("feed:category");
        return Arrays.asList(catgries);
}

The argument to the #asList method may not be null.