If you you want to add the defaults namespaces :
<namespace-allowed>http://www.w3.org/1999/xhtml</namespace-allowed>
<namespace-allowed>http://www.w3.org/2000/svg</namespace-allowed>
<namespace-allowed>http://www.w3.org/1998/Math/MathML</namespace-allowed>
<namespace-allowed>http://www.w3.org/XML/1998/namespace</namespace-allowed>
<namespace-allowed> </namespace-allowed>

if you want to add the namespace "" you have to write :
<namespace-allowed> </namespace-allowed>
instead of :
<namespace-allowed></namespace-allowed>.

(Use a single space).
If you don't the entry is not add to the hashset of the allowed namespaces.