HPCC-12300 remote copy creates ambiguous xpath

Fix 'ECL' property duplication bug.

Add code to prevent empty 'ECL' property creation.

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>
Have you looked at what other properties there are and whether they should be add or set.

I would have expected in most cases we would want to merge any subtrees with existing ones.