NetconfMessageTransformUtil.toFilterStructure() fails for leaf instance identifiers

This utility method is used by NETCONF plugin to create a filter expression and write it out. Unfortunately this functionality relied leaf nodes having a null value, which would in turn be recognized and ignored by codecs.

Null-valued leaves are no longer available, which breaks the ability to create filters for simple leaves, rendering sal-netconf-connector inoperable for requests to read individual leaves.