XmlProcessing: Fix warnings from the lxml module

In case you test for None using an instance associated with the lxml
module like this

    if obj:

the module will output "future warnings". This patch changes those
checks to:

    if obj is not None:

Reported-by: Ondrej Lichtner <olichtne@redhat.com>
Signed-off-by: Radek Pazdera <rpazdera@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>