HPCC-11002 Fix Regression in 5.0 dataset with xpath('Name')

Fix that any xpath that resulted in a xml nesting tag with a blank name would
have created an unnamed tag of the form <></>.

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Merge pull request #5521 from afishbeck/xmlWriterBlankXpath

HPCC-11002 Fix Regression in 5.0 dataset with xpath('Name')

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>