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

This regression was introduced in 4.2.  

xpath('') is used to store mixed content when a tag with both attributes
and content are needed.

&lt;tag att='mixed'>content&lt;/tag>

This regression caused the content to be inside an extra tag.

&lt;tag att='mixed'>&lt;name>content&lt;/name>&lt;/tag>

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>