NetconfCapabilityMonitoringService does not support revisionless models

This is day-0 bug, where we are requiring modules to have a revision in order to fill it out in the key for schema list in ietf-netconf-monitoring.

This runs contrary to its specification:

For YANG data models, version is the value of the most
recent YANG 'revision' statement in the module or
submodule, or the empty string if no 'revision' statement
is present.

Fix all code which interacts with this model to correctly use/interpret an empty string as an absent revision.