HPCC-10423 Fix regression xpath('') shouldn't output xml tag

xpath('') is used to store mixed content when a tag with both attributes
and content are needed.

<tag att='mixed'>content</tag>

This regression caused the content to be inside an extra tag.

<tag att='mixed'><name>content</name></tag>

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Merge branch 'empty_xpath_out' into candidate-4.2.0

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>