Saxon XSLT and XQuery Processor

Saxon rejects the instruction <xsl:namespace name="">,
which is legitimate: it should create a namespace node
for the default namespace.

This masks another bug, which is that Saxon does not
detect the user error of creating a namespace node for
the default namespace when the element itself is in the
null namespace. If the element is in the null
namespace, then the default namespace (ie, the
zero-length-prefix) must refer to the null namespace,
because no other prefix can be used for this purpose.

Reported by Paul V. Biron.

Affects all 7.x releases.

Source code fixed for both bugs.

Test cases added: nspc43 and error020.