module-xml: XML generation fails if an SQL NULL value is present

[davidnich]

should be treated as NOTHING - ex with Qore 0.8.13.4:

$ qore -lxml -nX 'make_xml({"a": NULL})'
"<?xml version=\"1.0\" encoding=\"UTF-8\"?>
<a></a>
"

With the current Qore version:

$ qore -l xml -nX 'make_xml({"a": NULL})'
unhandled QORE System exception thrown in TID 1 at 2024-04-09 12:17:07.211817 Tue +02:00 (CEST) in make_xml() (<command-line>:1 (Qore builtin code))
MAKE-XML-ERROR: cannot serialize type 'NULL'
call stack:
  1: make_xml() (<command-line>:1 (Qore builtin code))