aalto.in.NsBinding#createDefaultNs uses null instead of XMLConstants.DEFAULT_NS_PREFIX

[arne-bdt]

Version: 1.3.3
com.fasterxml.aalto.in.NsBinding#createDefaultNs creates an NsBinding where the prefix is null where it should be XMLConstants.DEFAULT_NS_PREFIX.

This is particularly problematic in conjunction with com.fasterxml.aalto.in.FixedNsContext#getNamespaceURI, which does not allow the prefix argument to be null.
So it is impossible to retrieve any URI for the default namespace from a FixedNsContent.