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>
no need to .get() like this. Linked/Owned have operator -> overloaded and it's cleaner to use them. And StringBuffer has const char *  operator overload, so you can write:
```
if (distributedTarget)
    distributedTarget->setECL(recStru);
```