[XML-SIG] empty prefix

[Sylvain Thenault]

One more problem with None vs '' for namespaces prefix:
 
it appears when using pDomlette with default namespaces (xmlns='...'), the
prefix of elements in default namespaces is '' instead of None. As
consequence, if you PrettyPrint the domtree, the default namespaces
attributes will appears twice...

This can easily be fixed by replacing '' with None in
xlm.dom.ext.SplitQName

-- 
Sylvain Thenault

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