Saxon XSLT and XQuery Processor

The instruction:
<xsl:element name="{$x}"/>

fails if $x contains no namespace prefix.

The error message is "namespace prefix has not been
declared".

Applies to Saxon 7.3 only.

I have fixed the source code; this bug is sufficiently
severe that I will probably issue a patch release to
fix it.

Test case added: nspc42. (I was rather shocked to
discover that none of my existing test cases covered
this area.)

The fix adds the following code at the end of
net.sf.saxon.instruct.NamespaceContext#getURIforPrefix():

if (prefix.equals("") && useDefault) {
// use the "default default namespace" -
namely ""
return "";
} else {
return null;
}