AttributeDefinition options should return null when not present

OSGi Compendium 7.0.0, 105.14.2.18 and 105.14.2.19 specify that if the
function returns null, there are no option values or labels available.

Currently, these get set to String[0] because the options map.size()
returns 0.

Should be fixed (I think) by https://github.com/apache/felix-dev/pull/49