Create NodeContext for all child nodes at beginning and
check if they are valid, either it has a valid target or
it contains valid nodes, so that we only export valid
node later.

Change-Id: I660d99011eb57ddc79f727455fce0be8876e8b17
Reviewed-on: https://gerrit.libreoffice.org/59892
Tested-by: Jenkins
Reviewed-by: Mark Hung <marklh9@gmail.com>