NullPointerException on empty config sections

[helgenlechner]

See http://sourceforge.net/p/geoserver/mailman/message/32452487/

A NullPointerException is generated when the config.yaml contains an empty section (e.g. a !legends block without any further configuration -- see http://pastebin.com/iw4AJtnF), even though it is a valid YAML file.

GeoServer version 2.5.1, plugin version print-lib-2.0-SNAPSHOT-20140410