This problem is related to
And it sounds like an immediate workaround is to reset the property value
using " " instead of "". The root of the problem is allowing property values
to be null-valued in the first place, and then using a serialization mechanism
(XML) that can't differentiate between null and empty-string.
Since we already allow the user to specify "All", maybe the real solution to this
problem (given the limitations of o.a.h.conf.Configuration and XML) is to also
allow the user to to specify "None".