BUG-1275: do not copy arrays in XmlDocumentUtilsforEachChild()

ImmutableList.Builder() has a nasty habit of performing poorly for
anything that is larger than 2 elements. Let's elide it by following
smarts:

- check if the list of children is empty
- pre-allocate an arraylist, thus copying the array at most once

Change-Id: I08825e15fba92e256bd415c192d3f30d52d6d8eb
Signed-off-by: Robert Varga <rovarga@cisco.com>