Fixed userdata plugin return value for Python3
Fixed userdata plugin return value for Python3

The comparison fails when ret_value is None in Python3.
This adds an extra check so that None will not be compared with an
integer.

Change-Id: Iddc6bbf42fc3215248d9187725a329181f2ba2fb

cmin764/cloudbase-init,alexpilotti/cloudbase-init,stackforge/cloudbase-init,CropCircleSys/bsd-cloudinit,whiteear/cloudbase-init,bincentvaret/bsd-cloudinit,stefan-caraiman/cloudbase-init,ader1990/cloudbase-init,chialiang-8/cloudbase-init,pellaeon/bsd-cloudinit,pellaeon/bsd-cloudinit,openstack/cloudbase-init